This commit redesigns how GC references work in the C API. previously
`wasmtime_{any,extern}ref_t` were both opaque pointers in the C API
represented as a `Box`. Wasmtime did not, however, provide the ability
to deallocate just the `Box` part. This was intended to be handled with
unrooting APIs but unrooting requires a `wasmtime_context_t` parameter,
meaning that destructors of types in other languages don't have a
suitable means of managing the memory around the
`wasmtime_{any,extern}ref_t` which might lead to leaks.

This PR takes an alternate approach for the representation of these
types in the C API. Instead of being an opaque pointer they're now
actual structures with definitions in the header file. These structures
mirror the internals in Rust and Rust is tagged to ensure that changes
are reflected in the C API as well. This is similar to how
`wasmtime_func_t` matches `wasmtime::Func`. This enables embedders to
not need to worry about memory management of these values outside of the
manual rooting otherwise required.

The hope is that this will reduce the likelihood of memory leaks and
otherwise not make it any harder to manage references in the C API.

prtest:full