Synchronize Rayon Max Threads with MT Executor

[th4s]

In order to prevent deadlocks it is important that the number of parallel rayon threads is synchronized with the MT executor's number of threads.

One approach to do this would be to use local ThreadPools which are owned by MTContext.
It is important that there is a way to inject different types of threadpools to support browser environments.

[sinui0]

RReverser/wasm-bindgen-rayon#18

[sinui0]

Closing, instead we've moved away from putting logical threads on the global rayon pool. Each logical thread has it's own system thread. See #187