Should we define GC-related `VM` types in `wasmtime-environ`?

[fitzgen]

We could probably even move the VM*Header and VMGcRef definitions into wasmtime-environ since VMGcRef is always 32 bits regardless of the target pointer size.

Originally posted by @fitzgen in #13107 (comment)

[fitzgen]

One thing to note is that the current VMDrcHeader and VMCopyingHeader types are defined in a module with visibility where they can only be used by the correct collector, and we have some unsafe trait implementations that rely on this visibility, and would instead become something like // SAFETY: nothing uses these when they shouldn't, which is much less satisfying.

But maybe is worth it to be able to just use offset_of! instead of defining constants manually and adding tests to assert that the constants match the actual struct's field's offsets?