Implement the payout Curve

[jellegerbrandy]

So the idea is that bounties are paid out according to a curve determined by (a) ta given percentage and (b) a maximum payout

This curve is described here:
https://docs.google.com/document/d/157koANfmBxzsyX3MQvDioGo8xfQHfiH0uZorEGNP10Q/edit

In that document, a payout curve is proposed that depends on two arguments: a severity of the bug S which is a number 0 <= S <= 1 , and a maximum payout C. So the amount that will be paid to the hacker is a function F(S, M) given by:

S * C * (1-exp(-V/C))

One problem with this is that solidity does not implement exponential curves natively, and so implementation is complex and relatively expensive.

An alternative is to implement a much simpler curve:

S * min(V, C)

Maximum expressed in dollar values, so we need an oracle :)

[jellegerbrandy]

OR we can use https://github.com/paulrberg/prb-math