Implement protocol fees

[PaulRBerg]

As agreed in #145, we should implement protocol fees. The idea is to have a mapping that maps token addresses to UD60x18 numbers , which would be used in the create functions to calculate a fee amount that gets accrued as protocol revenues. Consequently, there will also have to be a new mapping for storing the protocol revenues (mapping token addresses to revenue amounts), and another function that permits the contract owner to claim the revenues. Speaking of which, the owner should be implemented via the PRBContracts implementation of Ownable, because this implementation is faster than OpenZeppelin's (it doesn't inherit from Context and it uses custom errors instead of revert reason strings).

Note that the protocol fees should be stored in the comptroller.