Add multi-element wildcards to tuples and tuple-structs

[brson]

In #5830 we decided that all multi-element wildcards in patterns should be represented as .., not *, or .._, but also that tuple and tuple-struct patterns should support ... Add that support - there's a test case in src/test/run-pass/ignore-all-the-things.rs.

[eddyb]

The #10124 VG proposal includes ..binding in value lists - function arguments, tuples, tuple structs, enum variants etc. - with an optional binding (capturing multiple values in a tuple), a limited version of that can be added without a complete VG implementation.

[reem]

Triage: we now use .. to mark "the rest" of something during a pattern match, so I think this can be closed.

[steveklabnik]

Agreed.