* 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*'.
* 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.
* 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.
