Fully enable Winch in the differential fuzzer

[alexcrichton]

This commit fully enables usage of Winch in the differential fuzzer against all other engines with no special cases. I attempted enabling winch for the other fuzzers as well but Winch doesn't currently implement all methods for generating various trampolines required so it's currently only limited to the differential fuzzer.

This adds Winch as an "engine" and additionally ensures that when configured various wasm proposals are disabled that Winch doesn't support (similar to how enabling wasmi disables proposals that wasmi doesn't support).

This does reduce fuzzing of Winch slightly in that the reference-types proposal is completely disabled for Winch rather than half-enabled where Winch doesn't implement externref operations yet but does implement funcref operations. This, however, enables integrating it more cleanly into the rest of the fuzzing infrastructure with fewer special cases.

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[saulecabrera]

Thanks for working on this!

This does reduce fuzzing of Winch slightly in that the reference-types proposal is completely disabled for Winch rather than half-enabled where Winch doesn't implement externref operations yet but does implement funcref operations.

I think this is fine, we're starting the work on reference types, so we'd be able permanently enable fuzzing for funcref and externref soon-ish.

Given this change, I've taken the liberty to add myself to oss-fuzz (google/oss-fuzz#11602), as I'd like to monitor more closely any Winch fuzz bugs. Let me know if you're ok with that change.

[alexcrichton]

Let me know if you're ok with that change.

Definitely!