Allow truncated RawDevice::read_frame()

- `RawDevice::read_frame()` succeeds with a receive buffer smaller than the frame.
- Introduce `RxCursor` and `TxCursor` to track/update RX/TX buffer pointers making
  it easier to reason about functions such as `read_frame` / `write_frame`.

src/lib.rs

@@ -4,6 +4,7 @@
 #![doc = include_str!("../README.md")]
 
 pub mod bus;
+mod cursor;
 mod device;
 mod host;
 pub mod net;