BitArray additions, modifications

I needed some bitset code for a personal project. Your code was the most complete, but I did need to make some changes.  I listed all the changes in the changelog file.  Feel free to use any of them you like.

There's still some work to be done. I didn't implement all the "find clear" functions, I didn't try 64-bit mode, and the test programs need to be merged.

Thanks,
I think this line should be:

    bitarr->capacity_in_words = MAX(8, roundup2pow(new_num_of_words));

Since we only call realloc to increase allocation by a power of 2.