I believe this trait is no longer necessary since `WasmExternType` has
been moved inside the `wasmer` crate itself (this crate).
See https://github.com/wasmerio/wasmer-reborn/pull/121.
`cargo check` and `cargo test` agrees.
Also, this trait is annoying to implement abstraction over
`NativeFunc` since it is not re-exported outside of the crate.
Because `Func` needs an access to the runtime API (`wasmer-runtime`)
to trap properly, either we move parts of `wasmer-runtime` to
`wasm-common`, or we move parts of `wasm-common` into `wasmer`. I
decided to go with the second approach since `wasmer` is the only
crate to use `HostFunction` & co. It's not “common” by definition, and
it's way easier (for the moment).
`NativeFunc::call` is about 8% faster in the case we're benchmarking
by avoiding allocating a vector for the params / returns, instead we
do logic to determine which is larger and use that, conditionally
copying it back to the rets array if needed.
