refactor: create a usb backend trait

[louib]

This commit creates a Backend trait that encapsulates all the functions that are specific to a backend (rusb or nusb). This has the following advantages:

• All the backend-specific code is out of src/lib.rs and moved into the usb module and submodules.
• The number of cfg compilation directives is reduced drastically
• It will be easier to change the backend dynamically by changing which trait object is used as a backend
• It will also be easier to unit test the non-backend part of the codebase, by creating a MockBackend.

I'm not able to implement the dynamic backend feature at the moment, since I need to find a way to deal with the requirements for trait objects in Rust. This can definitely happen in a future PR.

[louib]

Re-exporting the Device struct to avoid introducing a breaking change.