MINOR: tools: implement functions to look up the nth bit set in a mask

MINOR: tools: implement functions to look up the nth bit set in a mask

Function mask_find_rank_bit() returns the bit position in mask <m> of
the nth bit set of rank <r>, between 0 and LONGBITS-1 included, starting
from the left. For example ranks 0,1,2,3 for mask 0x55 will be 6, 4, 2
and 0 respectively. This algorithm is based on a popcount variant and
is described here : https://graphics.stanford.edu/~seander/bithacks.html.