store: Make '%store-monad' an alias for '%state-monad'.

* guix/store.scm (define-alias): New macro.
  (%store-monad, store-return, store-bind): Define as aliases of the
  corresponding %STATE-MONAD part.
  (store-lift, text-file, interned-file): Return STORE as a second
  value.
  (run-with-store): Use 'run-with-state'.
* guix/packages.scm (set-guile-for-build, package-file): Return STORE as
  a second value.
* guix/monads.scm: Remove part of the module commentary.