Add benchmark with compile-time buffer sizes

[mgeier]

rtrb doesn't (yet?) support storing the buffer on the stack, but maybe it's interesting to compare other const-generic ring buffers.

For reference, this contains measurements with the good old rtrb::RingBuffer, just using a const capacity, even though the capacity isn't actually const in the implementation.

As always, there's also a corresponding violin plot:

[boranby]

I ran with cargo criterion but I couldn't get this plot generated. Is there anything I miss?

[mgeier]

I normally use cargo bench (possibly with a few options, like --bench two_threads_const or -- eager-pop). I have never tried cargo criterion.

[mgeier]

IIRC, I wanted to try it out some time ago, but then it looked unmaintained (bheisler/cargo-criterion#64) and I didn't bother.

[boranby]

Violin: