gexp: 'gexp-inputs' returns a list of <gexp-input> records.

This slightly reduces memory allocation.

* guix/gexp.scm (lower-inputs): Expect a list of <gexp-input> rather
than a list of tuples.
(lower-reference-graphs)[tuple->gexp-input]: New procedure.
Use it.
(gexp-inputs): Return a list of <gexp-input> rather than a list of
tuples.
* tests/gexp.scm (gexp-input->tuple): New procedure.
("one input package")
("one input package, dotted list")
("one input origin")
("one local file")
("one local file, symlink")
("one plain file")
("two input packages, one derivation, one file")
("file-append")
("file-append, output")
("file-append, nested")
("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.

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