Code Quality Improvements

[reyalabs]

• from an audit tag: /// @Audit is this precise, have there been instances where the aave rate is negative?
• from an audit tag: // @Audit - move docs to IRateOracle. Add additional parameter to use cache and implement cache (clean up outstanding issues with documentation related to rate oracle functionality.
• from an audit tag: // @Audit - how is the returned rate encoded? Floating rate?
• Unused imports are removed
• If/when introducing user defined types we should sanitise our various math/number libraries to avoid duplication. E.g. wad and PRBMathSD59x18 are both doing the same thing. *10^18 → PRBMathSD59x18.fromInt(number)
• Better to use seconds everywhere and convert to wad when needed, rather than the other way around; Better to use non-wad timestamps, but if using wad we should use Wad suffix on timestamp params.
• Get rid of all commented out code
• consider making some of the internal functions public and implement them if necessary (goes in line with the idea of needing a lot of test contracts when we can just make some of the functions of the core public)
• Introduce underscore prefix for param names and internal function names. This sort of convention can make the code easier to read.

[CyclopsRex]

I think we should work through these, but bias ourselves strongly towards making no/minimal code changes in light of the code now being frozen for audit. Deleting unused code or adding docs is probably OK.