"Downgrade" threads support to tier 2, disable fuzzing

[alexcrichton]

This commit is borne out of a fuzz bug that was opened recently. The fuzz bug specifically has to do with fallout from #12022, specifically SharedMemory being used to allocated instead of Memory. In this situation the resource limiter is no longer consulted meaning that shared memories bypass this and aren't caught by OOM checks. This is currently by design because SharedMemory instances don't know which resource limiter to hook into per-store.

More generally though the implementation of wasm threads, while workable in Wasmtime, has a number of known relatively large deficiencies. These were not resolved prior to ungating the wasm proposal (that's on me) but nevertheless the quality of implementation is not quite up to "tier 1 par" with the rest of what Wasmtime offers. Given that I'm proposing that threads is downgraded to tier 2 for now which means, primarily, that we won't issue CVEs for issues with it. The proposal is still on-by-default and usable-by-default, but my hope is to reflect the current level of quality in Wasmtime with this adjustment.

This commit shuffles around some documentation of wasm proposals to split it into tier 1/2/3 instead of on/off-by-default and then adds a column for whether the proposal is on-by-default.

[cfallin]

No objections from me at least; not sure if we want to discuss in the Wasmtime meeting or if anyone else has any input on the decision...

[alexcrichton]

Good point yeah, let's discuss in a meeting

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[alexcrichton]

Ok updated this from today's discussion in the Wasmtime meeting namely:

• The threads proposal is still downgraded to tier 2
• The WebAssembly threads feature continues to be enabled by default
• Creation of shared memories is now disabled by default

Notably there's not actually any means of exercising the threading/problematic parts of the API, namely SharedMemory. That being said single-threaded atomics are still allowed in wasm which should work just fine.

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