Increase default precision

[EVODelavega]

The addition of the PowPrecisionNegativeExponent variable in version 1.4 changes the behaviour of Pow significantly. In my use-case, I rely on Pow to convert amounts to and from a human-readable format and the same amount in the asset precision. When dealing with crypto (e.g. ETH), 18 decimal places is the de-facto norm.

Having this package default to a seemingly arbitrary precision of 16, especially for negative exponents makes little sense, defaults of 18 means this package can be used without having to rely on messy things like changing global variables to tweak the behaviour in an init function

[yordis]

@mwoss any opportunity to review this one? I just came across the same situation in the crypto space