* guix/gexp.scm (<local-file>): New record type.
(local-file): New procedure.
(local-file-compiler): New compiler.
(gexp->sexp) <struct? thing>: Handle the case where 'lower' returns a
file name.
(text-file*): Update docstring.local-file doc
* tests/gexp.scm ("one local file", "gexp->derivation, local-file"): New
tests.
* doc/guix.texi (G-Expressions): Mention local files early. Document
'local-file'. Update 'text-file*' documentation.