Remove benchmarks sublibrary.

Due how `cabal-install` works, all components are always solved for, and this causes unnecessary dependencies.

In case of `splitmix` it causes a dependency loop, so `splitmix` cannot test itself (or it should drop dependency on `vector` in tests).

Move the benchmarks into separate package, or fold into the benchmarks executable.

I'm not sure why benchmarks-O2 sublibrary is public in the first place, it isn't mentioned in the changelog.