gexp: Separate "compilers" for origins and packages from the core.

* guix/gexp.scm (<gexp-compiler>): New record type.
  (%gexp-compilers): New variable.
  (register-compiler!, lookup-compiler): New procedures.
  (define-gexp-compiler): New macro.
  (origin-compiler, package-compiler): New compilers.
  (lower-inputs): Remove clauses for 'origin?' and 'package?'.  Add
  clause with 'lookup-compiler' instead.
  (lower-references): Likewise.
  (gexp-inputs)[add-reference-inputs]: Likewise.
  (gexp->sexp)[reference->sexp]: Likewise.