gexp: Export 'gexp-input' constructor.

* guix/gexp.scm (<gexp-input>)[gexp-input]: Rename to...
  [%gexp-input]: ... this.  Adjust callers accordingly.
  (gexp-input): New procedure.
  (gexp-inputs)[add-reference-inputs]: When the input is a list, check
  whether each item is already 'gexp-input?' and to not rewrap those.
  (gexp-outputs)[add-reference-output]: Likewise.
  (gexp->sexp): Likewise.
* tests/gexp.scm ("input list splicing + gexp-input +
  ungexp-native-splicing"): New test.