Use worker threads

[lhstrh]

There is a new API in Node that allows for worker threads (these are different from Web Workers): https://nodejs.org/api/worker_threads.html

It looks like we can use these to exploit parallelism in reactor programs (like the C, C++, and Rust target already do).

[lhstrh]

A direction that is probably more promising, is to compile one of our other runtime implementations (C, C++, or Rust) down to Web Assembly and use that under the hood.