Fix externref/anyref ownership in C/C++ API

[alexcrichton]

This commit is a follow-up to #11514 which was discovered through failing tests in the wasmtime-py repository when updating to Wasmtime 37.0.0. Effectively a combination of bugs in the Rust API meant that it wasn't possible to use externref or anyref bindings correctly. The Rust changes in this commit are:

• wasmtime_val_unroot correctly drops the value now as opposed to effectively being a noop from before (typo of using as_externref vs from_externref).
• wasmtime_{anyref,externref,val}_t now have a Drop implementation in Rust to correctly drop them if a value in Rust is dropped. This is required to correctly manage memory in the wasmtime_func_{call,new} implementations, for example.
• wasmtime_{anyref,externref,val}_clone no longer have an unnecessary context parameter.
• wasmtime_{anyref,externref,val}_unroot no longer have an unnecessary context parameter.

Changes in the C/C++ APIs are:

• Result::{ok,err}_ref APIs were added in addition to the preexisting rvalue accessors.
• Loading/storing typed arguments now has an overload for const T& and T&& which behaves differently. Notably transferring ownership for T&& and not for const T&. This means that passing parameters when calling a wasm function uses const T&, but passing results from a host import uses T&&.
• TypedFunc::call now uses const Params& instead of Params to explicitly specify it doesn't modify the parameters and forces using the const T& store method.
• Store::gc is now a convenience method for store.context().gc()
• ExternRef, AnyRef, and Val now have ownership semantics and destructors. This matches the spirit of GC: replace ManuallyRooted with OwnedRooted. #11514 for Rust but models it in C++ as well. This required filling out move/copy constructors/assignments.
• The explicit ExternRef now takes std::any instead of T.
• Minor issues related to ownership are fixed in Val bindings.

Valgrind was used to ensure that there were no leaks for the test suite which additionally resulted in a number of *_delete calls being added to tests using the C API (accidental omissions).

The original goal of this change was to be a patch release for 37.0.1 to enable updating wasmtime-py to the 37.0.x releases of Wasmtime. In the end though the changes here were broad enough that I no longer feel that this is a good idea, so wasmtime-py will be skipping the 37 version of Wasmtime.

[alexcrichton]

Once this lands on main I'll backport the merged commit to the release-38.0.0 branch as well

[cfallin]

Carrying over my review from the GHSA -- thanks again for catching this and fixing it!