Major changes:
- Move to Rust 2018 edition #125
- Remove dependency on boxfnonce #121
- Reworked error handling #130
- Added a higher-level AuthenticatorService that can use multiple backends #123
- Changed the C API to use the new AuthenticatorService #123
- Added a Status channel for backends #122
- Now obtaining HID report sizes from the descriptors #12
- Add authenticator USB and Firmware details to the C API #93