Support native threads properly

[mattwparas]

Tracking issue for native threads

Right now, threads are created by creating a new runtime instance on another thread, where the entire runtime environment is copied and moved over, which is error prone since some values cannot be moved.

Error reporting is also poor on threads - they can error on creation and during runtime, and it will be difficult to know.

It also isn't particularly documented - so this is at least somewhere where progress on it can be documented.

[mattwparas]

Native thread support landed in #124 - documentation is coming. They are akin to rackets places, however perhaps a bit more ergonomic to get started with. There are surely bugs that exist 😄