Desired QoL changes for 0.15

[ralfbiedert]

The following changes I'd like to land in 0.15, help wanted

• [ ] Infer prefix in #[ffi_service] and make it optional
• [ ] Add sample how to use feature flags to disable Interoptopus unless generating bindings
• [ ] Improve the surrogate type situation, e.g., by instead relying on transmutable surrogates. Needs some thought.
• [ ] Maybe struct methods #29 (depends on how much work they are)
• [ ] Maybe implement #27, or rule out why they are a bad idea
• [ ] Make sure void service methods don't need annotations