Merge branch 'master' into core-updates

Conflicts:
	gnu-system.am

monads: Add 'text-file*'.

* guix/monads.scm (text-file*): New procedure.
* tests/monads.scm ("text-file*"): New test.
* doc/guix.texi (The Store Monad): Change example since the previous one
  would erroneously fail to retain a reference to Coreutils.  Document
  'text-file*'.

monads: Fix 'anym'.

* guix/monads.scm (anym): Fix successful case.
* tests/monads.scm ("anym"): New test.

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'.

monads: Allow resolution of a monad's bind/return at expansion time.

* guix/monads.scm (<monad>): Turn in a raw SRFI-9 record type.
  (define-monad): New macro.
  (with-monad): Add a case for when MONAD is a macro.
  (identity-return, identity-bind, store-return, store-bind): Inline.
  (%identity-monad, %store-monad): Use 'define-monad'.
* tests/monads.scm ("monad?"): New test.

Add (guix monads).

* guix/monads.scm: New file.
* tests/monads.scm: New file.
* Makefile.am (MODULES): Add guix/monads.scm.
  (SCM_TESTS): Add tests/monads.scm.
* doc/guix.texi (The Store Monad): New node.
  (The Store): Reference it.