gexp: Add <gexp-input>.

* guix/gexp.scm (<gexp-input>): New record type.
  (gexp-inputs)[add-reference-inputs]: Adjust clauses to expect
  <gexp-input> objects.
  (gexp-outputs)[add-reference-output]: Likewise.
  (gexp->sexp)[reference->sexp]: Likewise.
  (canonicalize-reference): Remove.
  (gexp)[escape->ref]: Use 'gexp-input' for all the references.
  Remove use of 'canonicalize-reference'.