Feature flag to disable rayon for benchmarking purposes

[rj00a]

While benchmarking Valence, the graph produced by cargo flamegraph is very difficult to understand due to rayon polluting the output (see this issue). Ideally a solution will be implemented upstream, but until then we'll have to come up with our own workaround.

I suggest adding a feature flag to valence named sequential. When enabled, all uses of rayon iterators internally will be replaced with their sequential counterparts. This flag should have no influence on the public API.

One idea I had was to make a rayon_shim module which could simply be glob imported. Then we won't have to sprinkle #[cfg(feature = "sequential")] all over the place.

[rj00a]

Decided that this was too annoying to maintain. RAYON_NUM_THREADS=1 gets close enough.