This extends a bit-vector from anything that produces individual bits.
This .extend() call is the second-slowest possible way to append bits into a bit-vector, faster only than calling iter.for_each(|bit| bv.push(bit)). DO NOT use this if you have any other choice.
If you are extending a bit-vector from the contents of a bit-slice, then you should use .extend_from_bitslice() instead. That method is specialized to perform upfront allocation and, where possible, use a batch copy rather than copying each bit individually from the source into the bit-vector.