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).
