r = a & b under 2s complement semantics. r may alias with a or b.
Asserts that r has enough limbs to store the result. If a or b is positive, the upper bound is @min(a.limbs.len, b.limbs.len)
. If a and b are negative, the upper bound is @max(a.limbs.len, b.limbs.len) + 1
.