CAN FD support for Linux

[optlink]

This PR has three major changes:

1. LinuxCan is now generic, accepting CanSocket or CanFdSocket.
2. LinuxCan will now properly convert ErrorKind::WouldBlock to nb::Error::WouldBlock on receive.
3. receive on CoreNode now loops until all available transfers are handled instead of handling one at a time.

This enables cleaner async support with Tokio and CAN FD support on Linux.

[samcrow]

Thanks for contributing. This looks good. Please add an item to the changelog in the Unreleased section.

[optlink]

Done