gexp: Move the package and origin compilers to (guix packages).

From now own, (guix packages) depends on (guix gexps); it was the other
way around now.  This means that (guix packages) code can use gexps.

* guix/gexp.scm (origin-compiler, package-compiler): Remove.
  (default-guile-derivation): New procedure.
  (gexp->derivation): Use it instead of 'default-guile' +
  'package->derivation'.
* guix/packages.scm (default-guile-derivation): New procedure.
  (package-compiler, origin-compiler): New variables.
* doc/guix.texi (G-Expressions): Mention extensibility.

gexp: Separate "compilers" for origins and packages from the core.

* guix/gexp.scm (<gexp-compiler>): New record type.
  (%gexp-compilers): New variable.
  (register-compiler!, lookup-compiler): New procedures.
  (define-gexp-compiler): New macro.
  (origin-compiler, package-compiler): New compilers.
  (lower-inputs): Remove clauses for 'origin?' and 'package?'.  Add
  clause with 'lookup-compiler' instead.
  (lower-references): Likewise.
  (gexp-inputs)[add-reference-inputs]: Likewise.
  (gexp->sexp)[reference->sexp]: Likewise.

gexp: Remove special meaning of forms (PACKAGE OUTPUT) in ungexp.

* guix/gexp.scm (gexp-inputs)[add-reference-inputs]: Remove clause for
  inputs of the form (PACKAGE OUTPUT).
  (gexp->sexp)[reference->sexp]: Likewise.
* tests/gexp.scm ("input list splicing"): Change 'list' to 'gexp-input'
  for glibc:debug.
  ("text-file*"): Likewise for %bootstrap-guile:out.
  ("input list splicing + gexp-input + ungexp-native-splicing"): Remove,
  now redundant.

gexp: Export 'gexp-input' constructor.

* guix/gexp.scm (<gexp-input>)[gexp-input]: Rename to...
  [%gexp-input]: ... this.  Adjust callers accordingly.
  (gexp-input): New procedure.
  (gexp-inputs)[add-reference-inputs]: When the input is a list, check
  whether each item is already 'gexp-input?' and to not rewrap those.
  (gexp-outputs)[add-reference-output]: Likewise.
  (gexp->sexp): Likewise.
* tests/gexp.scm ("input list splicing + gexp-input +
  ungexp-native-splicing"): New test.

gexp: Add <gexp-input>.

* guix/gexp.scm (<gexp-input>): New record type.
  (gexp-inputs)[add-reference-inputs]: Adjust clauses to expect
  <gexp-input> objects.
  (gexp-outputs)[add-reference-output]: Likewise.
  (gexp->sexp)[reference->sexp]: Likewise.
  (canonicalize-reference): Remove.
  (gexp)[escape->ref]: Use 'gexp-input' for all the references.
  Remove use of 'canonicalize-reference'.

gexp: Rename <output-ref> to <gexp-output>.

* guix/gexp.scm (<output-ref>): Rename to...
  (<gexp-output>): ... this.  Adjust constructor/accessor names and
  users accordingly.

Merge branch 'core-updates'.

gexp: Make sure 'gexp-outputs' removes duplicate outputs.

Fixes a regression introduced in f9efe56.

* guix/gexp.scm (gexp-outputs): Add call to 'delete-duplicates'.
* tests/gexp.scm ("output list, combined gexps, duplicate output"): New
  test.

gexp: Aggregate outputs of compound gexps.

* guix/gexp.scm (gexp-outputs)[add-reference-output]: Recurse into
  lists.
* tests/gexp.scm ("output list + ungexp-splicing list, combined gexps"):
  New test.

Merge branch 'master' into core-updates

gexp: Add #:graft? parameter to 'gexp->derivation'.

* guix/gexp.scm (gexp->derivation): Add #:graft? parameter and honor it.
* tests/gexp.scm ("gexp->derivation vs. grafts"): New test.
* doc/guix.texi (G-Expressions): Update 'gexp->derivation'
  documentation.

gexp: Implement 'imported-modules' & co. using 'gexp->derivation'.

* guix/derivations.scm (imported-files): Keep private.
  (%imported-modules, %compiled-modules, build-expression->derivation):
  Mark as deprecated.
  (imported-modules, compiled-modules): Remove.
* guix/gexp.scm (%mkdir-p-definition): New variable.
  (imported-files, search-path*, imported-modules, compiled-modules):
  New procedures.
* tests/derivations.scm ("imported-files"): Remove.
* tests/gexp.scm ("imported-files", "gexp->derivation #:modules"): New
  tests.

Merge branch 'master' into core-updates

gexp: Add #:allowed-references parameter to 'gexp->derivation'.

* guix/gexp.scm (lower-references): New procedure.
  (gexp->derivation): Add #:allowed-references and honor it.
* tests/gexp.scm ("gexp->derivation #:allowed-references",
  "gexp->derivation #:allowed-references, disallowed"): New tests.
* doc/guix.texi (G-Expressions): Update 'gexp->derivation' doc.

monads: Move '%store-monad' and related procedures where they belong.

This turns (guix monads) into a generic module for monads, and moves the
store monad and related monadic procedures in their corresponding
module.

* guix/monads.scm (store-return, store-bind, %store-monad, store-lift,
  text-file, interned-file, package-file, package->derivation,
  package->cross-derivation, origin->derivation, imported-modules,
  compiled, modules, built-derivations, run-with-store): Move to...
* guix/store.scm (store-return, store-bind, %store-monad, store-lift,
  text-file, interned-file): ... here.
  (%guile-for-build): New variable.
  (run-with-store): Moved from monads.scm.  Remove default value for
  #:guile-for-build.
* guix/packages.scm (default-guile): Export.
  (set-guile-for-build): New procedure.
  (package-file, package->derivation, package->cross-derivation,
  origin->derivation): Moved from monads.scm.
* guix/derivations.scm (%guile-for-build): Remove.
  (imported-modules): Rename to...
  (%imported-modules): ... this.
  (compiled-modules): Rename to...
  (%compiled-modules): ... this.
  (built-derivations, imported-modules, compiled-modules): New
  procedures.
* gnu/services/avahi.scm, gnu/services/base.scm, gnu/services/dbus.scm,
  gnu/services/dmd.scm, gnu/services/networking.scm,
  gnu/services/ssh.scm, gnu/services/xorg.scm, gnu/system/install.scm,
  gnu/system/linux-initrd.scm, gnu/system/shadow.scm, guix/download.scm,
  guix/gexp.scm, guix/git-download.scm, guix/profiles.scm,
  guix/svn-download.scm, tests/monads.scm: Adjust imports accordingly.
* guix/monad-repl.scm (default-guile-derivation): New procedure.
  (store-monad-language, run-in-store): Use it.
* build-aux/hydra/gnu-system.scm (qemu-jobs): Add explicit
  'set-guile-for-build' call.
* guix/scripts/archive.scm (derivation-from-expression): Likewise.
* guix/scripts/build.scm (options/resolve-packages): Likewise.
* guix/scripts/environment.scm (guix-environment): Likewise.
* guix/scripts/system.scm (guix-system): Likewise.
* doc/guix.texi (The Store Monad): Adjust module names accordingly.

Merge branch 'master' into core-updates

monads: Rewrite 'text-file*' using gexps.

* guix/monads.scm (text-file*): Move to...
* guix/gexp.scm (text-file*): ... here.  Rewrite using gexps.
* tests/monads.scm ("text-file*"): Move to...
* tests/gexp.scm ("text-file*"): ... here.

gexp: Add #:module-path parameter to 'gexp->derivation'.

* guix/gexp.scm (gexp->derivation): Add #:module-path parameter, and
  pass it to 'imported-modules' and 'compiled-modules'.
* doc/guix.texi (G-Expressions): Adjust accordingly.

Merge branch 'master' into core-updates

gexp: 'gexp->script' returns a script that can easily be compiled.

* guix/gexp.scm (gexp->script): Produce an 'eval-when' form around
  assignments of %load-path and %load-compiled-path.