pull: Move build code to (guix build pull).

* guix/build/pull.scm: New file.
* Makefile.am (MODULES): Add it.
* guix/scripts/pull.scm (unpack): Use it.

pull: Compile guix/ files before gnu/ files.

* guix/scripts/pull.scm (unpack)[builder]: Build files under guix/
  before files under gnu/.

pull: Compile files in parallel.

* guix/scripts/pull.scm (unpack)[builder](compile-file*): Remove.
  (call-with-process, p-for-each): New procedures.  Use them to compile
  files in parallel.

pull: Limit memory usage when compiling.

Reported by Arne Babenhauserheide <arne.babenhauserheide@kit.edu>.

* guix/scripts/pull.scm (unpack)[builder](compile-file*): Change to run
  'compile-file' in a child process.  This limits memory usage; before
  that memory usage was proportional to the number of files to compile.

derivations: Use more keyword parameters for 'build-expression->derivation'.

* guix/derivations.scm (build-expression->derivation): Turn 'system' and
  'inputs' into keyword parameters.
  Adjust callers accordingly.
* gnu/system/linux.scm, gnu/system/vm.scm, guix/build-system/cmake.scm,
  guix/build-system/gnu.scm, guix/build-system/perl.scm,
  guix/build-system/python.scm, guix/build-system/trivial.scm,
  guix/download.scm, guix/packages.scm, guix/profiles.scm,
  guix/scripts/pull.scm, tests/derivations.scm, tests/guix-build.sh,
  tests/monads.scm, tests/store.scm, tests/union.scm: Adjust users of
  'build-expression->derivation' and 'derivation-expression'
  accordingly.
* doc/guix.texi (Derivations): Adjust 'build-expression->derivation'
  documentation accordingly.
  (The Store Monad): Likewise for 'derivation-expression'.

Merge branch 'master' into core-updates

pull: Compile modules correctly regardless of the compilation order.

* guix/scripts/pull.scm (unpack)[builder]: Work around
  <http://bugs.gnu.org/15602>.

pull: Simplify the builder.

* guix/scripts/pull.scm (unpack)[builder]: Use 'copy-recursively' and
  'copy-file' instead of 'file-system-fold'.

pull: Add '--url' option.

* guix/scripts/pull.scm (%default-options): Add 'tarball-url' pair.
  (%options, show-help): Add '--url'.
  (guix-pull): Honor it.

Merge branch 'core-updates'

Merge branch 'master' into core-updates

pull: Adjust to 'derivation' API change.

Fixes <http://bugs.gnu.org/15428>.
Reported by Cyrill Schenkel <cyrill.schenkel@gmail.com>.

* guix/scripts/pull.scm (guix-pull): 'unpack' returns a single value.

utils: 'find-files' returns a sorted list.

* guix/build/utils.scm (find-files): Sort the result lexicographically.
* guix/scripts/pull.scm (unpack): Don't sort the result of 'find-files'.

Merge branch 'master' into core-updates

pull: Compile files in the lexicographic order.

* guix/scripts/pull.scm (unpack): Print the name of the file being
  compiled.  Sort the names of files to compile.

ui: Add `args-fold*' and use it.

* guix/ui.scm (args-fold*): New procedure.
* guix/scripts/build.scm, guix/scripts/download.scm,
  guix/scripts/gc.scm, guix/scripts/hash.scm, guix/scripts/import.scm,
  guix/scripts/package.scm, guix/scripts/pull.scm,
  guix/scripts/refresh.scm: Use `args-fold*' instead of `args-fold'.

pull: Use `download-to-store'.

* guix/scripts/pull.scm (download-and-store): Remove.
  (unpack): Use `download-to-store' instead.

pull: Switch to the cgit URL.

* guix/scripts/pull.scm (%snapshot-url): Switch to the cgit URL, given
  that the Hydra one is not currently available.

pull: Switch to the cgit URL.

* guix/scripts/pull.scm (%snapshot-url): Switch to the cgit URL, given
  that the Hydra one is not currently available.

Merge branch 'master' into core-updates

Conflicts:
	Makefile.am
	gnu/packages/base.scm