C++ API: Make `wasmtime::Store::gc` return a `Result` (#12439)

* C++ API: Make `Store::gc()` return a `Result`

* Handle GC errors in C++ example programs

* clang format

Fix externref/anyref ownership in C/C++ API (#11799)

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

This commit is a follow-up to #11514 which was discovered through
failing tests in the wasmtime-py

Fix externref/anyref ownership in C/C++ API (#11799)

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

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 #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.

* Run `clang-format`

prtest:full

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c-api: Bring wasmtime-cpp into this repository (#10582)

* c-api: Bring wasmtime-cpp into this repository.

`wasmtime-cpp/include/wasmtime.hh` -> `/crates/c-api/include/`
`wasmtime-cpp/tests/*` -> `/

c-api: Bring wasmtime-cpp into this repository (#10582)

* c-api: Bring wasmtime-cpp into this repository.

`wasmtime-cpp/include/wasmtime.hh` -> `/crates/c-api/include/`
`wasmtime-cpp/tests/*` -> `/crates/c-api/tests/`
`wasmtime-cpp/examples/*` -> `/examples/`

See discussion in #10566 for more context.

* Make tests disabled by default prtest:full

* Mention C++ API in the c-api README

* Enable C++ only for tests

* Add *.hh files to doxygen

This requires disabling warnings on undocumented, as `wasm.hh` has
undocumeted structures

Also fix urls in `wasmtime.hh`

* Re-enable `WARN_IF_UNDOCUMENTED`

This required excluding `wasm.hh`, `wasmtime::detail` namespace, and some
macros.

Also documented two overloads to `Func::call()`

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