Revert stabilization of feature(never_type).

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 `()`.

src/libstd/lib.rs

@@ -275,6 +275,7 @@
 #![feature(macro_reexport)]
 #![feature(macro_vis_matcher)]
 #![feature(needs_panic_runtime)]
+#![feature(never_type)]
 #![feature(exhaustive_patterns)]
 #![feature(nonzero)]
 #![feature(num_bits_bytes)]