Implement the wasi-threads specification in Wasmtime.
Note: this crate is experimental and not yet suitable for use in multi-tenant embeddings. As specified, a trap or WASI exit in one thread must end execution for all threads. Due to the complexity of stopping threads, however, this implementation currently exits the process entirely. This will work for some use cases (e.g., CLI usage) but not for embedders. This warning can be removed once a suitable mechanism is implemented that avoids exiting the process.