This commit adds methods to RawDevice that enable interrupt-driven
operation. The enable_interrupt() method sets up SIMR so that
socket-level (internal) interrupts on Socket 0 cause chip-level
(external) interrupts (and as a convenience also sets S0_IR as
required). The disable_interrupt() method reverses those changes.
The clear_interrupt() method acknowledges all interrupts and is
intended to be called from the interrupt handler (or from thread mode
soon afterwards).
There is no change to existing functionality or operation if
enable_interrupt() is never called.
I did see PR#34 before filing this, but that change is focused on
TCP and UDP sockets, and my use case is MACRAW mode.
Tested on a W5500-Pico-EVB board with the RP2040 successfully receiving
and acting on active-low GPIO interrupts from W5500 via the INTn signal
on W5500 pin 36.
- `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`.
