* Introduce a new API that allows notifying that a Store has moved to a new thread
* Add backlink to documentation, and mention the new API in the multithreading doc;
* Document guidance around multithreading and Wasmtime
This commit writes a page of documentation for the Wasmtime book to
serve as guidance for embedders looking to add multithreading with
Wasmtime support. As always with any safe Rust API this reading is
optional because you can't mis-use Wasmtime without `unsafe`, but I'm
hoping that this documentation can serve as a point of reference for
folks who want to add multithreading but are confused/annoyed that
Wasmtime's types do not implement the `Send` and `Sync` traits.
Closes#793
* I can type
