concat_idents! is not as useful as it could be, due to macros only being
allowed in limited places, and hygiene, so lets feature gate it until we
make a decision about it.
cc #13294
This updates a number of ignore-test tests, and removes a few completely
outdated tests due to the feature being tested no longer being supported.
This brings a number of bench/shootout tests up to date so they're compiling
again. I make no claims to the performance of these benchmarks, it's just nice
to not have bitrotted code.
Closes#2604Closes#9407
