monads: Add a template and specialization mechanism for monadic procedures.

* guix/monads.scm (%templates, %template-instances): New variables.
(register-template!, register-template-instance!): New procedures.
(template-directory, define-template): New macro.
(foldm, sequence, anym): Define using 'define-template'.  Avoid replace
ellipses with dots.
(mapm): Likewise, but do not use 'foldm'.
* guix/store.scm: Add 'template-directory' invocation.