As an alternative to `ThreadPoolBuilder::build()` and `build_global()`,
the new `spawn()` and `spawn_global()` methods take a closure which will
be responsible for spawning the actual threads. This is called with a
`ThreadBuilder` argument that provides the thread index, name, and stack
size, with the expectation to call its `run()` method in the new thread.

The motivating use cases for this are:
- experimental WASM threading, to be externally implemented.
- scoped threads, like the new test using `scoped_tls`.