* guix/gexp.scm (<parameterized>): New record type.
(with-parameters): New macro.
(compile-parameterized): New gexp compiler.
* tests/gexp.scm ("with-parameters for %current-system")
("with-parameters for %current-target-system")
("with-parameters + file-append"): New tests.
* doc/guix.texi (G-Expressions): Document it.