Merge branch 'master' into core-updates

tests: Skip tests that would hit the shebang length limitation.

* tests/gexp.scm (shebang): New variable.
  Skip "gexp->script" when SHEBANG is longer than 127 chars.
* guix/gexp.scm (gexp->script): Add comment on the issue.

Merge branch 'master' into core-updates

gexp: Add support for 'origin?' objects in 'ungexp' forms.

* guix/gexp.scm (lower-inputs, gexp-inputs, gexp->sexp,
  canonicalize-reference): Add 'origin?' case.
* guix/monads.scm (origin->derivation): New procedure.
* tests/gexp.scm ("one input origin"): New test.

Add (guix gexp).

* guix/gexp.scm: New file.
* tests/gexp.scm: New file.
* Makefile.am (MODULES): Add guix/gexp.scm.
  (SCM_TESTS): Add tests/gexp.scm.
* doc/guix.texi (Derivations): Add #:inputs in 'derivation' example.
  Mark 'build-expression->derivation' as deprecated, refer to
  "G-Expressions".  Remove paragraph about code strata.
  (G-Expressions): New node.