Accurately compute operations on u128

[kianenigma]

Add minimal structures to the runtime arithmetic primitives to support most importantly

fn multiply_by_rational<N: Mul + Div>(a: N, b: N, c: N) -> Option<N>

The only tricky case will be u128. Rest is trivial.

This is needed to make all the computations in Staking and phragmen accurate for any possible balance type.

This will be a follow up of #3456