[interpreter] Threading

[rossberg]

This adds threading support to the interpreter:

• Add threads to evaluator and script runner, with randomised execution.
• Add thread and wait script commands for spawning and joining threads.
• Add either result pattern for testing non-deterministic results.
• Extend JS harness with new functions thread and wait (not yet fleshed out).
• Add a trivial test.
• Added command line flags for setting non-determinism random seed.

This includes scripting functionality as discussed in #163. However, I decided to make sharing more explicit with (sharing ...) annotations in a thread definition, because it's more readable, avoids funny errors, and it's probably gonna be easier to implement in the JS converter that way. See test for how it's used.

Sorry for the large PR, this required more refactoring than I anticipated. In particular, the whole script runner had to effectively be transformed from big step to small step. Similarly, Eval.invoke and Eval.init needed to become small steps.

[SuspendedPhan]

Sorry for the noise, but just wanted to say seriously thanks for working on this! golang said they would implement threads once this moves to stage 3 :D

-- excited adopter of wasm

[conrad-watt]

Is it possible to lift the either combinator so that it applies to lists of values, rather than individual values (see comment on multi-value below)?

[conrad-watt]

LGTM

I see that wait/notify implementations are stubbed out - would these need refactorings to implement?

[rossberg]

I see that wait/notify implementations are stubbed out - would these need refactorings to implement?

Good question, I don't know. I hope not.