Avoid vector allocations in wasm->host calls (#3294)

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.

crates/wasmtime/Cargo.toml

@@ -30,7 +30,6 @@ rustc-demangle = "0.1.16"
 cpp_demangle = "0.3.2"
 log = "0.4.8"
 wat = { version = "1.0.36", optional = true }
-smallvec = "1.6.1"
 serde = { version = "1.0.94", features = ["derive"] }
 bincode = "1.2.1"
 indexmap = "1.6"