compat: directly assign bytes_from_uint = chr

compat: directly assign bytes_from_uint = chr

This is significantly faster than the indirection and
seems to reduce the runtime of the test suite by about
3% on my machine with python 2 (76.121s -> 73.725s,
but I tried only once which is clearly not enough.)

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>