[bench] separate comparative benchmarks

[fwbrasil]

Problem

Kyo's benchmarks are currently all in the same module and package. As we evolve the library, we'll likely need to introduce benchmarks that aren't meant for comparing different effect systems. Having all benchmarks in the same package is also inconvenient when you want to execute only the comparative ones for example.

Solution

Move comparative benchmarks to a separate kyo.bench.arena package. Benchmarks that don't follow the comparative pattern with sync/fork benchmarks were kept in the kyo.bench package.