Remove benchmarks sublibrary.

[phadej]

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.