fn popCount(r: *Mutable, a: Const, bit_count: usize) void
r = @popCount(a) with 2s-complement semantics. r and a may be aliases.
Assets the result fits in r. Upper bound on the number of limbs needed by r is calcTwosCompLimbCount(bit_count).
r
calcTwosCompLimbCount(bit_count)
r: *Mutable,
a: Const,
bit_count: usize,