gexp: Native inputs of nested gexps are properly accounted for.

gexp: Native inputs of nested gexps are properly accounted for.

Previously, 'gexp-native-inputs' would not return the native inputs of
nested gexps.  For example, this:

  (gexp-native-inputs #~(foo #$#~(bar #+coreutils)))

would return '().

* guix/gexp.scm (gexp-inputs)[add-reference-inputs]: In the
non-recursive cases, check whether N? and NATIVE? are the same, and act
accordingly.
[native-input?]: Remove.
Fold over all of (gexp-references exp).
* tests/gexp.scm ("ungexp + ungexp-native, nested, special mixture"):
New test.
* tests/gexp.scm ("input list splicing + ungexp-native-splicing"): Pass
 #:native? #t to 'gexp-input'.