pub type Digit = i8;
Type used to represent w-NAF digits.
For a window of size w non-zero w-NAF digits are odd and have magnitude at most 2^(w-1) - 1 and lie within {-(2^(w-1)-1), 2^(w-1)-1}.
w
2^(w-1) - 1
{-(2^(w-1)-1), 2^(w-1)-1}