gexp: Catch and report non-self-quoting gexp inputs.

Previously we would, for example, generate build scripts in the store;
when trying to run them, we'd get a 'read' error due to the presence
of #<foo> syntax in there.

* guix/gexp.scm (gexp->sexp)[self-quoting?]: New procedure.
[reference->sexp]: Check whether the argument in a <gexp-input> box is
self-quoting.  Raise a '&gexp-input-error' condition if it's not.
* tests/gexp.scm ("lower-gexp, non-self-quoting input"): New test.