Compile with a stable Rust

[woodruffw]

We should aim to support compilation and use with a stable Rust release, to avoid encountering nightly bugs and depending on unstable APIs.

Some items that need to be handled:

• Reverie currently uses the unstable std::test and #[bench]. We should probably use criterion instead.
• Reverie uses #[feature(stdsimd)] since the MMX intrinsics were never stabilized. Once they're removed in favor of SSE2 and AVX2, we can remove this #[feature]. (gf2/domain{8,64}: MMX -> SSE #4)

[woodruffw]

Criterion looks like a little bit of a mess. Instead, we can keep using #[bench] and move all benchmarks under benches/ to avoid polluting the main source tree with unstable uses.

[woodruffw]

Merging #4 and #7 will allow some progress by unmooring us from the current nightly-2020-08-01 and allowing us to use any relatively recent nightly.

[woodruffw]

Just revisiting: we now support recent nightlies, but #[bench] is still unstable. So we're stuck on nightly builds for the foreseeable future here.