xor method
Implementation
void xor(BitArray other) {
if (_size != other._size) {
throw ArgumentError("Sizes don't match");
}
for (int i = 0; i < _bits.length; i++) {
// The last int could be incomplete (i.e. not have 32 bits in
// it) but there is no problem since 0 XOR 0 == 0.
_bits[i] ^= other._bits[i];
}
}