Prefix FFI callback function and struct names with the crate name

[bendk]

We do this for FFI functions, but we don't do it for structs or callback function types. This could lead to name collisions if two crates export traits with the same name.

UniFFI structs / callback functions that are always defined like ForeignFuture and ForeignFutureFree should not be prefixed IMO. It's cleaner to have a single definition of those. This maybe means we need to continue using if-guards like swift does, or maybe we could implement a method that combines/dedups the FFI definitions from multiple ComponentInterface instances.