GHSA-vvp9-h8p2-xwfc: Wasmtime: Memory leak in C API with `externref` and `anyref` types

Impact Wasmtime 37.0.0 and 37.0.1 have memory leaks in the C/C++ API when using bindings for the anyref or externref WebAssembly values. This is caused by a regression introduced during the development of 37.0.0 and all prior versions of Wasmtime are unaffected. If anyref or externref is not used in the C/C++ API then embeddings are also unaffected by the leaky behavior. The wasmtime Rust crate is unaffected by this leak. Development of Wasmtime 37.0.0 included a refactoring in Rust of changing the old ManuallyRooted<T> type to a new OwnedRooted<T> type. This change was integrated into Wasmtime's C API but left the C API in a state which had memory leaks. Additionally the new ownership semantics around this type were not reflected into the C++ API, making it leak-prone. A short version of the change is that previously ManuallyRooted<T>, as the name implies, required manual calls to an "unroot" operation. If this was forgotten then the memory was still cleaned up when the wasmtime_store_t itself was destroyed eventually. Documentation of when to "unroot" was sparse and there were already situations prior to 37.0.0 where memory would be leaked until the store was destroyed anyway. All memory, though, was always bound by the store, and destroying the store would guarantee that there were no memory leaks. In migrating to OwnedRooted<T> the usage of the type in Rust changed. A manual "unroot" operation is no longer required and it happens naturally as a destructor of the OwnedRooted<T> type in Rust itself. These new resource ownership semantics were not fully integrated into the preexisting semantics of the C/C++ APIs in Wasmtime. A crucial distinction of OwnedRooted<T> vs ManuallyRooted<T> is that the OwnedRooted<T> type allocates host memory outside of the store. This means that if an OwnedRooted<T> is leaked then destroying a store does not release this memory and it's a permanent memory leak on the host. This led to a few distinct, but related, issues arising: -