DRAFT: Needs more tests and more testing.

* guix/gexp.scm (gexp)[lookup-binding, generate-bindings]
[syntax-uid, alpha-rename]: New procedures.
Call 'alpha-rename' before doing anything else.
* tests/gexp.scm ("hygiene, eval", "hygiene, define")
("hygiene, shadowed syntax", "hygiene, quote"): New tests.