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.