The BitmaskImpl class had functions for finding
the least significant bit set in word and array
of words (ffs(), find_first(), find_next()).

Now functions for finding also the most significant set bit
are added.

fls() - find most significant bit in Uint32, index 0-31
ctz() - count trailing (least sign.) zeros, equal to ffs()
clz() - count leading (most sign.) zeros. That is the
        number of bits preceding the most significant set
        bit. That is: fls(x) + clz(x) = 31 if x!=0.

All these function are undefined for zero words.

find_last() and find_prev() are added to find the most 
significant set bits in an array of words, corresponding
to find_first() and find_next().

find_last()/find_prev() allows one bit outside word array,
better would probably be to exclude the given start bit,
but that would mean update all current uses of find_last().