Add bits package with add function

[hextrust-0]

This PR adds the bits package with an Add function.
The function can check if the sum of two integers overflows or not.
An example of this function's use is in a modular addition function, as shown below:

func modAdd(x uint256, y uint256, mod uint256) uint256 {
	zero := uint256(0)
	sum, overflow := bits.Add(x, y, zero)
	if overflow != 0 || sum > mod {
		sum -= mod
	}

	return sum
}

Note that above example modAdd only works when mod is greater than 2^128 .