guix package: '-s' sorts packages by name, then by version.

Before that it would sort them by name only, so the order in which two
packages with the same name but a different version would appear was
non-deterministic.

Reported by Tomáš Čech <sleep_walker@gnu.org>.

* guix/scripts/package.scm (find-packages-by-description)[version<?]:
  New variable.
  Change the 2nd argument to 'sort' to use 'string-compare' and resort
  to 'version<?' when P1 and P2 have the same name.

guix package: '-s' displays different packages that have the same location.

Before that, 'guix package -s foobarbaz' would display only one package
when several match but they have the same location (which is common when
using 'inherit'.)

The original rationale was given at
<http://lists.gnu.org/archive/html/bug-guix/2013-01/msg00280.html> but
it was arguably misguided because it led to "real" packages being
hidden.

Reported by Tomáš Čech <sleep_walker@gnu.org>.

* guix/scripts/package.scm (find-packages-by-description)[same-location?]:
  Remove.
  Remove call to 'delete-duplicates'.

ftp-client: Switch to binary mode before using the "SIZE" command.

* guix/ftp-client.scm (ftp-size): Add '%ftp-command' call.

lint: Report details about FTP errors.

* guix/scripts/lint.scm (probe-uri) <'ftp>: Pass more information about
  failures alongside 'ftp-response.
  (validate-uri) <ftp-response>: Handle it, and adjust "not reachable"
  message accordingly.

lint: Change misleading variable name.

* guix/scripts/lint.scm (probe-uri) <'ftp>: Rename 'port' to 'conn'.

substitute-binary: Fix recently-introduced regression.

* guix/scripts/substitute-binary.scm (%cache-url): Fix regression
  introduced in 41c45e7.

store: Remove debugging leftover.

* guix/store.scm (set-build-options): Remove leftover 'pk' call from
  41c45e7.

store: Add preliminary support for client-supplied substitute URLs.

* guix/store.scm (set-build-options): Rename #:binary-caches to
  #:substitute-urls.  Actually pass it in 'pairs' under the
  "substitute-urls" key.
* guix/scripts/substitute-binary.scm (%cache-url): Add comment for
  "untrusted-substitute-urls".

pk-crypto: Improve documentation of 'key-type'.

* guix/pk-crypto.scm (key-type): Improve docstring.

build: ruby: Set $GEM_HOME that matches Ruby's $GEM_PATH.

* guix/build/ruby-build-system.scm (install): Ignore the Ruby patch version
  when creating $GEM_HOME.

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.

packages: Move grafting parameter to (guix derivations).

* guix/packages.scm (%graft?, set-grafting): Move to...
* guix/derivations.scm: ... here.

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.

profiles: Use 'gexp-input' instead of two-element lists.

* guix/profiles.scm (package->manifest-entry): Use 'gexp-input' instead
  of two-element lists to denote specific package outputs.
  (manifest-inputs): Likewise.
  (profile-derivation)[info-dir]: Likewise.

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.

licenses: Add the Ms-PL.

* guix/licenses.scm (ms-pl): New variable.

scripts: environment: Improve error messages.

* guix/scripts/environment.scm (guix-environment): Wrap procedure body with
  error handling form.