Enum API with newtypes

[wmanley]

This is a tracking issue for the work I've been doing on enum based API.

The goal is to have an easier to use, more decoupled, more typesafe, more extensible and better documented API than is possible in languages with less powerful type-systems. I believe by exploiting Rust's type-system we can create a much nicer API than is possible in C (libfuse) or golang. This means using enums, newtypes, the the typestate pattern, ? based error handling, etc.

TODO

• [x] A type per request type #119
• [ ] Document every request type #140
• [ ] Check each u32/u64 in the API and decide if a newtype would be more appropriate - particularly around flags
• [x] #137
• [x] Response type #131
• [ ] Add serve_sync() API #136
• [ ] Add serve_async() API
• [ ] Add serve_multithreaded() API
• [ ] Port https://github.com/wmanley/ostree-oxide/tree/master/ostree-fuse to see how it looks
• [ ] Refactor Filesystem trait API into a module?
• [ ] Use typestate pattern to encapsulate Init: https://github.com/cberner/fuser/issues/91#issuecomment-809283993
• [ ] Make enum API public #136

[cberner]

Closing due to inactivity, but feel free to re-open if you're still working on this.