Compounding rewards calculator

[geoff-vball]

Context and scope
We should create a compounding rewards calculator so that validators are not incentivized to end their staking period and restake frequently.

Discussion and alternatives
Solidity only natively supports integer math. We may need a library for this, or would could probably get by with a reasonable approximation (ex: compound every X days).

Open questions
Can/should we allow the validator to update their stake weight on the p-chain based on their current compounded rewards?

[cam-schultz]

Closing in favor of #526. They both would have similar effects when claiming validation rewards on ending a validation, but 526 also unlocks much better UI that doesn't require evicting delegators before claiming rewards.