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.

Review by @adriaanm, @paulp.

In particular, I'm after suggestions about how to issue a positioned warning from `Infer` in a less clunky way.