Slightly improve crc_ccitt.
Slightly improve crc_ccitt.

By looking at Python bytecodes, it turns out that doing
a test-n-jump on a bool is faster than anything else,
and avoiding the global lookup of sys can be done outside
of the method, so do that to speed up crc_ccitt.

Signed-off-by: Chris Lalancette <281cd07d7578d97c83271fbbf2faddb83ab3791c@gmail.com>

clalancette/pyiso,clalancette/pycdlib