use fp3 container

[nikolaydubina]

using exposing decimal container is good so that other code that uses this money type can also work with coefficient and other numerics expressed in decimals of compatible type. such as if we have currency in fp3 and some other coefficients in fp3, then they can work together

however, this does not work for now. currencies leak precision with less that 3 units

then there is issue with amount is in fp2 and coefficient in fp3, leading to leaks of money precision

using fp2 will not help. as KRW and JPY will leak into fractions.

no simple way to address this for now. keeping dynamic precision for now. sigh.

for sake of currency precision, not exposing underlying decimal container.

keeping PR open for future