monads: Hide 'derivation-expression' and 'lower-inputs'.

* guix/monads.scm: Unexport 'lower-inputs' and 'derivation-expression'.
  (text-file*): Add comment about the switch to 'gexp->derivation'.
  (lower-inputs): Add comment about its doom.
  (derivation-expression): Likewise.
* guix/gexp.scm (lower-inputs*): Rename to...
  (lower-inputs): ... this.  Update callers.
* tests/monads.scm (derivation-expression): New procedure.
* doc/guix.texi (The Store Monad): Use 'gexp->derivation' instead of
  'derivation-expression'.  Remove documentation of
  'derivation-expression'.
* guix/ui.scm (read/eval): Use THE-ROOT-MODULE so that macros are
  properly expanded.
* tests/guix-build.sh: Use 'gexp->derivation' instead of
  'derivation-expression'.monads: Hide 'derivation-expression' and 'lower-inputs'.