When the version has type `Version`, we expect the user to give a
valid `Version` variant. Since the `Version` is basically a `uint8_t`,
the user is able to pass everything she wants, which can create an
undefined behavior on the Rust side.
To avoid such situation, the version has now type `c_uchar` (`unsigned
char` or `uint8_t` on C side —on most platforms). Then the `From`
trait is implemented on `Version`. In case the value is unbound,
`Version::Unknown` is returned.
This patch introduces 2 new functions:
* `wasmer_wasi_generate_import_object_for_version` and
* `wasmer_wasi_get_version`.
It mimics the current API provided by `wasmer_wasi`, nothing fancy
here. It's just a regular port to C/C++.
Because `wasmer_wasi::get_wasi_version` returns an option, and in
order to simplify the C/C++ API, `wasmer_wasi_get_version` can return
`Version::Unknown` in case of an error. It's up to the user to check
the version is valid (i.e. not unknown).
