SerialConnect: support sending raw files

Add a generic FileSender interface, which XModemSender implements. Add a
new RawSender which implements the same interface.

The RawSender currently blocks the application thread while sending,
which is not a good idea. Will rework this when I allow cancelling
transfers before they complete.

SerialConnect: implement XMODEM-CRC

This is a common extension to XMODEM, which replaces the checksum with a
more resilient CRC.
The receiver must explicitly enable this, so if the receiver doesn't
handle it, the traditional checksum is still used. Hence, this is
backwards-compatible with XMODEM.

SerialConnect: implement XMODEM send.