Path to Phase 4

The current main branch has become miserably out of date. We now have a rebased version of (most of) the threads specification living in a new branch (and I'm very grateful for @ioannad's invaluable help here). I've also passed over this branch to excise some long-standing bugs (e.g. https://github.com/WebAssembly/threads/issues/195) and other TODOs.

I'm using this issue to track my progress towards getting Threads phase 4-ready, since the only remaining bar facing the proposal's standardisation is full specification text. My intention is to start out doing this work in the upstream-rebuild branch, although I'm sure we'll have a conversation about how to update main at some point.

Core document TODOs:

• generalise binary encoding to allow LEBs
• give text descriptions for wait and notify
• describe behaviour of the wait' (suspend) administrative instruction
• fix tearing behaviour of float and SIMD non-atomics
• describe the behaviour of the concurrent host
  • todo: finish writing prose / evaluate alternative formalisations
  • todo: fully remove old host state particle
• extend configurations to hold multiple threads
• fix shared memory bounds checks in instantiation
• rebase in the axiomatic constraints of the relaxed memory model
  • todo: predicates should be coinductive
    • todo: work out why sphinx is rendering negative space weirdly
  • todo: auxiliary functions
  • todo: finish writing prose
• extend these constraints to describe wait/notify queue behaviour
• link the above constraints to the actions emitted in the operational semantics
  • todo: syntax for coinductives
• formatting - add line breaks where the PDF overflows

JS-API document TODOs:

• check with an expert whether I got the JS-API rebase correct
• integrate with growable SharedArrayBuffer
• final inline todos

Interpreter TODOs:

• rebase the interpreter into the new branch
  • syntax
  • single-thread semantics
  • fix cmpxchg wrapping behaviour (#195)
  • green threads and wait/notify implementations (#194)
• collect together the tests from the other WIP branches
• update .wast -> JS test generation tool to handle thread and wait

Misc TODOs:

• re-unify with main(?)
• update the spec render
• check whether any other documents need updating
• final inline TODO sweep

Start by reviewing the upstream-rebuild branch and the remaining unchecked items in the Core, JS-API, Interpreter, and Misc TODO lists. The issue names unfinished specification prose, wait/notify constraints, formatting, and .wast-to-JS test generation as areas to inspect. Done requires resolving the remaining TODOs and determining whether the updated work should be unified with main.