This continues #788 and literally removes the type from the public API
of the `wasmtime` crate, making it inaccessible to the outside world.
Now it's purely an implementation detail, yay!
* Only require `str` in `new_with_name`
It's a bit more idiomatic to have APIs require `&str` rather than
`String`, and the allocation doesn't matter much here since creating a
`Module` is pretty expensive anyway.
* Update a test