bitset: function for getting minimum index with bit set about mlib

[dmlary]

Wow, thanks! And thank you for the library as a whole!

[P-p-H-d]

Indeed bitset_ctz is missing (count trailing zero) which is the equivalent of roaring_bitmap_minimum (See https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html )
I prefer your first proposal to your second one(bitset_clear_lowest): doing bitset_ctz + bitset_set_at will be as efficient.

[P-p-H-d]

bitset_ctz is available in master.