Add Engine::precompile_compatibility_hash

[lann]

This method returns a Hash, the output of which can be used to index precompiled binaries from one Engine instance that can be deserialized by another Engine instance.

Closes #5802

[alexcrichton]

I think the documentation here may want to be a bit more nuanced because if two blobs are the same then you should be able to load between engines but if they're different they may still load correctly. For example different values of generate_address_map will produce different blobs but that's not checked when loading modules into engines.

I think it may be best to reword this as something along the lines of "if two blobs are equal artifacts from one engine are guaranteed to be loadable in another" or something like that.

[lann]

Yeah that's what I was trying to convey but "if" is ambiguous. I'll be more explicit about it.

[github-actions]

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[lann]

Finally getting back around to this, I realized that the cache system already generates a key like this, and sure enough this has exactly what I was looking for:

wasmtime/crates/wasmtime/src/module.rs

Lines 1141 to 1151 in 83d00fe

	/// This is a helper struct used when caching to hash the state of an `Engine`
	/// used for module compilation.
	///
	/// The hash computed for this structure is used to key the global wasmtime
	/// cache and dictates whether artifacts are reused. Consequently the contents
	/// of this hash dictate when artifacts are or aren't re-used.
	#[cfg(all(feature = "cache", compiler))]
	struct HashedEngineCompileEnv<'a>(&'a Engine);
	#[cfg(all(feature = "cache", compiler))]
	impl std::hash::Hash for HashedEngineCompileEnv<'_> {

Reusing this code as-is would require changing the function signature a bit, e.g.:

pub fn precompile_compatibility_hash(&self) -> impl std::hash::Hash {
    crate::module::HashedEngineCompileEnv(self)
}

[alexcrichton]

Sounds reasonable to me!

[jameysharp]

This looks sensible to me and appears to be exactly what Alex already said sounded good, so I'll go ahead and merge it. Thank you!