This commit is just covering the feature gate itself and the tests
that made direct use of `!` and thus need to opt back into the
feature.
A follow on commit brings back the other change that motivates the
revert: Namely, going back to the old rules for falling back to `()`.
Replace feature(never_type) with feature(exhaustive_patterns).
feature(exhaustive_patterns) only covers the pattern-exhaustives checks
that used to be covered by feature(never_type)
For the most part, the current code performs similarly, although it
differs in some particulars. I'll be nice to have these tests for
judging future changes, as well.
