@popCount with two’s complement semantics.
This returns the number of 1 bits set when the value would be represented in two’s complement with the given integer width (bit_count). This includes the leading sign bit, which will be set for negative values.
Asserts that bit_count is enough to represent value in two’s compliment and that the final result fits in a usize. Asserts that there are no trailing empty limbs on the most significant end, i.e. that limb count matches calcLimbLen()
and zero is not negative.