[SECURITY] Analysis of the impact of not using exponential math in the contract

[hellwolf]

Problem: Compound finance uses https://github.com/decentral-ee/rtoken-contracts/blob/master/compound/contracts/Exponential.sol to handle big integer number math, in rToken contract it is deliberately omitted for using less storage, simplicity or laziness :)

Acceptance Criteria:

• Give a report on to what extent the problem will be for not using the exponential math component.