gexp: 'gexp-inputs' returns both native and non-native inputs.

gexp: 'gexp-inputs' returns both native and non-native inputs.

This avoids double traversal of references and extra bookkeeping,
thereby further reducing memory allocations.

* guix/gexp.scm (lower-gexp): Include only one call to 'lower-inputs'.
(gexp-inputs): Remove #:native? parameter.
[set-gexp-input-native?]: New procedure.
[add-reference-inputs]: Use it.
(gexp-native-inputs): Remove.
* tests/gexp.scm (gexp-native-inputs): Remove.
(gexp-input->tuple): Include 'gexp-input-native?'.
("let-system")
("let-system, nested")
("ungexp + ungexp-native")
("ungexp + ungexp-native, nested")
("ungexp + ungexp-native, nested, special mixture")
("input list")
("input list + ungexp-native")
("input list splicing")
("input list splicing + ungexp-native-splicing")
("gexp list splicing + ungexp-splicing"): Adjust accordingly.