Add more functions for Ratio

[apopiak]

We will want to use Ratio in calculations.

Missing functions

• mul_int(Ratio, u128) -> u128 (multiply Ratiowith u128, returning 128) (make generic? probably not)
• invert(Ratio) -> Ratio (invert numerator and denominator)
• rounding_mul(Ratio, Ratio. Rounding) -> Ratio (multiply 2 ratios and round them down from (U256, U256) to Ratio
• full_mul(Ratio, Ratio) -> (U256, U256)