Support Swift func returns Opaque Swift

[chinedufn]

This commit adds the ability for a Swift function or method to return an
opaque Swift type.

For example, the following is now possible:

#[swift_bridge::bridge]
mod ffi {
    extern "Swift" {
        type SomeType;

        fn some_function(arg: SomeType) -> SomeType;
        fn some_method(&self, arg: SomeType) -> SomeType;
    }
}