This patch adds a new field in `Func`: `signature`. It contains the
signature of the host function.
For non-polymorphic host functions, the signature is computed from the
`Args` and `Rets` implementation parameters at compile-time.
For polymorphic host functions though, to be fully dynamic, the
signature given to `new_polymorphic` is used in `Func` as the correct
signature.
This non-publishable new crate contains a test suite for the
`wasmer-runtime-core` crate. So far, the test suite is rather small,
but it aims to be extended in a close future.
