Redesign the core/executor

[petr-tik]

As mentioned in #638 (comment), our core/executor relies on the scoped_pool crate, which looks abandoned over the last 2 years.

The Executor struct relies on a ThreadPool from scoped_pool but the comments mention Rayon's ThreadPool.

Using development activity as an indicator, Rayon looks like a better long-term dependency to use when building a core executor than scoped_pool.

Rayon provides a scope method to create a scope inside a ThreadPool. However, it requires the FnOnce to exhibit the Send trait.

This discussion outlines pros and cons of this approach and how other people suggest working around it.

If rayon doesn't work, we can roll our own on top of Crossbeam

[fulmicoton]

I'm generally ok with this as long as it works! :)