The low level popualtion counting function is now separated from the
BITCOUNT command implementation, so that the low level function can be
further optimized and eventually used in other contexts if needed.
All the general string operations are implemented in t_string.c, however
the bit operations, while targeting the string type, are better served
in a specific file where we have the implementations of the following
four commands and helper functions:
GETBIT
SETBIT
BITOP
BITCOUNT
In the future this file will probably contain more code related to
making the BITOP and BITCOUNT operations faster.