SI-6675 -Xlint arity enforcement for extractors

Extractor Patterns changed in 2.10.0 to implement
the letter of the spec, which allows a single binding
to capture an entire TupleN. But this can hide arity
mismatches, especially if the case body uses the
bound value as an `Any`.

This change warns when this happens under -Xlint.
SI-6675 Test new warning under -Xoldpatmat.

This commit should be discarded when merging to master.
Merge pull request #1888 from retronym/ticket/6675

SI-6675 -Xlint arity enforcement for extractors