* 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.