[mono] unify and vectorize implementation of decode_value metadata API

[kg]

We have multiple copies of the same decode_value function spread across the mono runtime. This PR unifies them all into a single implementation under metadata/, and then vectorizes it using clang vector builtins.

A basic benchmark on x64 using clang -O3 showed a 13% time reduction, and the generated wasm for this is pretty efficient, so I'm hoping it will be a small startup time win for any target where we enable it. It's hard to actually measure in practice locally though...

I verified that the vectorized implementation works correctly by comparing its output against the scalar version for all possible 5-byte sequences, so this should be a safe switch-over as long as there aren't endianness issues.

Pending stuff to fix for this PR:

• Enable SIMD for metadata on wasm the correct way
• Make certain that it's actually safe to swap all the other copies of the algorithm over to this one (at least one appears to have contained a bug)
• Is it safe to overrun when reading on other targets? POSIX rounds mmap up to page-sized units, and extra bytes off the end are zeroes, so it might be?

[kg]

cc @radekdoulik re our earlier conversation about simd in other parts of the runtime

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Draft Pull Request was automatically closed for 30 days of inactivity. Please let us know if you'd like to reopen it.