crc: add a faster, chainable version of CRC16/CCITT.

The existing version of crc16_ccitt() is actually CRC-16/AUG-CCITT and
gives different results to Linux, Contiki, and the CRC unit in the
SAM0 SOC.  This version matches Linux.

Note that this is an incompatible API change.

Signed-off-by: Michael Hope <mlhx@google.com>