emacs: Add 'guix-pull' command.

* emacs/guix-base.el (guix-pull): New command.
  (guix-update-after-pull, guix-after-pull-hook): New variables.
  (guix-restart-repl-after-pull, guix-update-buffers-maybe-after-pull): New
  procedures.
* emacs/guix-main.scm: Use (guix scripts pull) module.
* doc/emacs.texi (Emacs Commands): Document 'guix-pull' command.

doc: Add a note about optional GnuTLS dependency.

* README (Requirements): add a note about 'guix download''s GnuTLS dependency.
* doc/guix.texi (Requirements): Likewise.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

doc: Add cross-reference to the gnutls-guile manual.

* doc/guix.texi (Invoking guix download): Add cross-reference to the
  gnutls-guile manual.

emacs: Improve interface for working with multiple profiles.

Suggested by David Thompson, Ludovic Courtès and Mathieu Lirzin.

* emacs/guix-base.el (guix-profile-prompt): New procedure.
  (guix-set-current-profile): Use it.
  (guix-buffer-name-simple, guix-buffer-name-default, guix-buffer-name): New
  procedures.
  (guix-buffer-name-function, guix-profile): New variables.
  (guix-set-vars, guix-get-entries, guix-get-show-entries, guix-set-buffer,
  guix-history-call, guix-process-package-actions,
  guix-continue-package-operation-p, guix-delete-generations,
  guix-switch-to-generation): Add 'profile' argument.
* emacs/guix.el (guix-get-show-packages, guix-get-show-generations,
  guix-search-by-name, guix-search-by-regexp, guix-installed-packages,
  guix-obsolete-packages, guix-all-available-packages,
  guix-newest-available-packages, guix-generations, guix-generations-by-time):
  Likewise.
  (guix-last-generations): New command.
* emacs/guix-info.el: Adjust for using 'profile' argument where needed.
* emacs/guix-list.el: Likewise.
* doc/emacs.texi (Emacs Commands): Document 'guix-last-generations' and using
  "C-u" for commands.
  (Emacs Buffer Names): Document 'guix-buffer-name-function'.

doc: Replace "emacs" with "Emacs" in node names.

Suggested by Ludovic Courtès.

* doc/emacs.texi: Use "Emacs" in node names.

emacs: Add 'guix-generations-by-time' command.

* emacs/guix-main.scm (find-generations): Add finding generations by time.
* emacs/guix-utils.el (guix-read-date): New procedure.
* emacs/guix-base.el (guix-messages): Add new messages.
* emacs/guix.el (guix-generations-by-time): New command.
* doc/emacs.texi (emacs Commands): Document it.

guix lint: add the --checkers option.

* guix/scripts/lint.scm: add the "--checkers" option.
* doc/guix.texi (Invoking guix lint): Document it.
* tests/guix-lint.sh: New file
* Makefile.am (SCM_TESTS): Add it.

emacs: Add support for switching generations.

* emacs/guix-base.el (guix-switch-to-generation): New procedure.
* emacs/guix-info.el (guix-generation-info-insert-current): Insert button for
  switching to generation.
* emacs/guix-list.el (guix-generation-list-switch): New procedure.
* doc/emacs.texi (emacs List buffer, emacs Info buffer): Document switching
  generations.

guix package: Add '--switch-generation' option.

* guix/scripts/package.scm (switch-to-generation): New procedure.
  (switch-to-previous-generation): Use it.
  (guix-package): Adjust for '--switch-generation' option.
* tests/guix-package.sh: Test it.
* doc/guix.texi (Invoking guix package): Document it.

scripts: Add 'environment' command.

* guix/scripts/environment.scm: New file.
* Makefile.am (MODULES): Add it.
* doc/guix.texi ("Invoking guix environment"): New node.

doc: Add 'operating-system' reference.

* doc/guix.texi (Using the Configuration System): Add xref to
  "operating-system Reference".
  (operating-system Reference): New section.
  (Networking Services): Add xref to it.

doc: Use @deftp for 'build-machine'.

* doc/guix.texi (Daemon Offload Setup): Use @deftp for 'build-machine'.

daemon: Add '--substitute-urls' option.

* nix/nix-daemon/guix-daemon.cc (GUIX_OPT_SUBSTITUTE_URLS): New macro.
  (GUIX_OPT_NO_BUILD_HOOK, GUIX_OPT_GC_KEEP_OUTPUTS,
  GUIX_OPT_GC_KEEP_DERIVATIONS): Renumber.
  (options): Add '--substitute-urls'.
  (parse_opt): Honor it.
  (main): Add 'settings.set' call for the default "substitute-urls"
  value.
* guix/scripts/substitute-binary.scm (daemon-options,
  find-daemon-option): New procedures.
  (%cache-url): Define based on the "substitute-urls" daemon option.
* doc/guix.texi (Invoking guix-daemon): Document '--substitute-urls'.
  (Substitutes): Mention it.

monads: Add 'mbegin'.

* guix/monads.scm (mbegin): New macro.
* tests/monads.scm ("mbegin"): New test.
* doc/guix.texi (The Store Monad): Document it.

doc: Document 'glib-or-gtk-build-system'.

* doc/guix.texi (Build System): Document 'glib-or-gtk-build-system'.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

services: Add '%facebook-host-aliases'.

* gnu/services/networking.scm (%facebook-host-aliases): New variable.
* doc/guix.texi (Networking Services): Document it.

doc: Mention optional dependency on Guile-JSON.

* README (Requirements): Add Guile-JSON.
* doc/guix.texi (Requirements): Likewise.

emacs: Add support for deleting generations.

* doc/emacs.texi (emacs List buffer): Mention new key bindings.
* emacs/guix-base.el (guix-delete-generations): New procedure.
* emacs/guix-info.el (guix-generation-info-insert-number): Use it.
* emacs/guix-list.el (guix-generation-list-mark-delete,
  guix-generation-list-execute): New procedures.
* emacs/guix-main.scm (delete-generations*): New procedure.

build-system: Introduce "bags" as an intermediate representation.

* guix/build-system.scm (<build-system>)[build, cross-build]: Remove.
  [lower]: New field.
  (<bag>): New record type.
  (make-bag): New procedure.
* guix/packages.scm (bag-transitive-inputs, bag-transitive-build-inputs,
  bag-transitive-host-inputs, bag-transitive-target-inputs,
  package->bag): New procedures.
  (package-derivation): Use it; use the bag, apply its build procedure,
  etc.
  (package-cross-derivation): Likewise.
* gnu/packages/bootstrap.scm (raw-build, make-raw-bag): New procedure.
  (%bootstrap-guile): Use them.
* guix/build-system/trivial.scm (lower): New procedure.
  (trivial-build, trivial-cross-build): Remove 'source' parameter.  Pass
  INPUTS as is.
  (trivial-build-system): Adjust accordingly.
* guix/build-system/gnu.scm (%store, inputs-search-paths,
  standard-search-paths, expand-inputs, standard-inputs): Remove.
  (gnu-lower): New procedure.
  (gnu-build): Remove 'source' and #:implicit-inputs? parameters.
  Remove 'implicit-inputs' and 'implicit-search-paths' variables.  Get
  the source from INPUT-DRVS.
  (gnu-cross-build): Likewise.
  (standard-cross-packages): Remove call to 'standard-packages'.
  (standard-cross-inputs, standard-cross-search-paths): Remove.
  (gnu-build-system): Remove 'build' and 'cross-build'; add 'lower'.
* guix/build-system/cmake.scm (lower): New procedure.
  (cmake-build): Remove 'source' and #:cmake parameters.  Use INPUTS and
  SEARCH-PATHS as is.  Get the source from INPUTS.
* guix/build-system/perl.scm: Likewise.
* guix/build-system/python.scm: Likewise.
* guix/build-system/ruby.scm: Likewise.
* gnu/packages/cross-base.scm (cross-gcc): Change "cross-linux-headers"
  to "linux-headers".
  (cross-libc)[xlinux-headers]: Pass #:implicit-cross-inputs? #f.
  Likewise.  In 'propagated-inputs', change "cross-linux-headers" to
  "linux-headers".
* guix/git-download.scm (git-fetch): Use 'standard-packages' instead of
  'standard-inputs'.
* tests/builders.scm ("gnu-build-system"): Remove use of
  'build-system-builder'.
  ("gnu-build"): Remove 'source' and #:implicit-inputs? arguments to
  'gnu-build'.
* tests/packages.scm ("search paths"): Adjust to new build system API.
  ("package-cross-derivation, no cross builder"): Likewise.
* doc/guix.texi (Build Systems): Add paragraph on bags.

build-system/cmake: Add #:build-type parameter, default to "RelWithDebInfo".

* guix/build-system/cmake.scm (cmake-build): Add #:build-type.  Pass it
  in BUILDER.
* guix/build/cmake-build-system.scm (configure): Add #:build-type
  parameter and honor it.
* doc/guix.texi (Build Systems): Document #:configure-flags and
  #:build-type for CMake.