Check usage of floating point calculations and u64 casting

Currently, in the payout calculation, there are a lot of places where U256 is cast to u64 and/or f64. This can cause both panics (for the u64 casting) and inaccuracies in the payout calculation (for the f64 casting).

One potential solution could be to use a decimal floating point library like https://github.com/alkis/decimal