Single-threaded subscription evaluation

[joshua-spacetime]

Description of Changes

Removes rayon from subscription evaluation and replaces its parallel iterators with sequential ones, the purpose of which is to optimize the common case of small updates where multithreading just adds overhead.

API and ABI breaking changes

None

Expected complexity level and risk

1

Just removes rayon and replaces into_par_iter() with into_iter().

Testing

• Bot test

[CLAassistant]

All committers have signed the CLA.

[gefjon]

Changes look good. I don't love having #[allow(unused)] code committed; it feels a bit too much like commented-out code. I'd rather just remove the thing we don't need any more, and we can add it back in down the line if/when it's useful again.