Support (and provide tests and examples for) multithreading #248

Now that WASI-SDK 34 has been released with wasm32-wasip3 support for pthreads via the Component Model's cooperative multithreading feature, we should be able to support multithreading in Python code. Alex Crichton has run the CPython test suite with multithreading enabled with a few patches. In theory, things should "just work" once we apply those patches to the CPython fork we're using in this project.

Note that #242 is a prerequisite for this.