Wasmtime: Add a gc cargo feature

[fitzgen]

This controls whether support for ExternRef and its associated deferred, reference-counting garbage collector is enabled at compile time or not. It will also be used for similarly for Wasmtime's full Wasm GC support as that gets added.

[fitzgen]

I didn't add a smoke test build without the gc cargo feature to CI, but I can do that if we decide we can afford the CI time.

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[alexcrichton]

Nice!

My main comments below are about perhaps shifting around #[cfg] to try to reduce it. That's my main worried with these sorts of features is that it can become difficult in the limit to ensure all the #[cfg] annotations are aligned so I'm hoping we can try to reduce them as much as possible. More-or-less I think that ExternRef and probably VMExternRef should have the same API no matter the feature set (modulo constructors) and that way consumers don't have to #[cfg] which should help centralize this.

We'll also definitely want this on CI and I'd recommend putting it around here

[fitzgen]

Okay, I've updated this PR to stub out more VMExternRef things and methods when GC is disabled at compile time and removed a bunch of cfgs from the rest of the code base. Not all of them in store.rs, as mentioned inline above. I think this is ready for another round of review.

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