Adding in thread_pool::spawn function

[Pauan]

This adds in a thread_pool::spawn function which can be used to run some Rust code on multiple Workers for multithreading.

This is the first step toward implementing #790

[iamalwaysuncomfortable]

This looks good to merge, one thing we will want however is to ensure we're writing some integration tests around spawning functions onto the threadpool.

[Pauan]

Absolutely, that will be in the next PR that actually makes use of the spawning.