hashsplit: replace join_bytes with cat_bytes

hashsplit: replace join_bytes with cat_bytes

Add a C cat_bytes that can concatenate two bytes objects with offsets
and extents.  This allows us to have the same implementation for
python 2 and 3, to drop another use of buffer(), and may be handy in
the future, particularly given the expense of getting a buffer offset
in python 3 (i.e. memoryview() adds about ~200 bytes).

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>