dev: Avoid vector allocations in wasm->host calls (#3294)
Pre-release
Pre-release
This commit improves the runtime support for wasm-to-host invocations for functions created with `Func::new` or `wasmtime_func_new` in the C API. Previously a `Vec` (sometimes a `SmallVec`) would be dynamically allocated on each host call to store the arguments that are coming from wasm and going to the host. In the case of the `wasmtime` crate we need to decode the `u128`-stored values, and in the case of the C API we need to decode the `Val` into the C API's `wasmtime_val_t`. The technique used in this commit is to store a singular `Vec<T>` inside the "store", be it the literal `Store<T>` or within the `T` in the case of the C API, which can be reused across wasm->host calls. This means that we're unlikely to actually perform dynamic memory allocation and instead we should hit a faster path where the `Vec` always has enough capacity. Note that this is just a mild improvement for `Func::new`-based functions. It's still the case that `Func::wrap` is much faster, but unfortunately the C API doesn't have access to `Func::wrap`, so the main motivation here is accelerating the C API.