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.