Make UD60x18-pow work with values lower than 1e18

[PaulRBerg]

The pow function in UD60x18 reverts when x is lower than 1e18, because the log2 function is used in an intermediary step, and so in-flight negative values are required.

However, it is possible to circumvent this limitation by using this mathematical trick suggested by @FoxDev12 in #176 (which is closed now, unfortunately):

x^y = 1/((1/x)^y)

In practice, the idea is to refactor the pow function to look something like this:

if (xUint > uUNIT) {
    result = exp2(mul(log2(x), y));
} else if (xUint == uUNIT) {
    result = UNIT;
} else {
    UD60x18 rX = div(UNIT, x);
    UD60x18 rResult = exp2(mul(log2(rX), y));
    result = div(UNIT, rResult);
}