Update to recent Wasmtime C API changes regarding values

[kpreisser]

Update to recent Wasmtime C API changes regarding values.

This includes updates for:

• bytecodealliance/wasmtime#8011
• bytecodealliance/wasmtime#8451
• bytecodealliance/wasmtime#8461

TODOs:

• [X] Allocating an externref can now fail (by wasmtime_externref_new returning false). With this PR, we will throw a WasmtimeException in that case. We need to check where that exception can be thrown, and whether we need to do any additional clean-up (e.g. when converting arguments for a function call).
• [x] Check whether we can avoid directly accessing the __private field of externs (which has been remaned in the C API, previously it was index).

Note: The anyref type is not yet supported; it was introduced by the GC Proposal. Support for this type can be added separately, after deciding how the API should look like.

Fixes part of #315 (the WASI part is fixed by #316).

[kpreisser]

I changed the cache in the Store from #235 to use separate ConcurrentDictionary instances for each type, to avoid having to explicitly access the __private field of the externs (instead the whole struct is now compared), and is a bit more type-safe since we don't need to use a cast.