This patch adds Tether support and creates a number of small gas optimizations. The goal is to reduce about 10-20K of gas on operations by simplying certain operations in the protocol, without significantly changing behavior. Specfically we:

1. Remove `checkTransferIn` as the now look at the effect of `transfer` to handle fees, which obviates the need for check transfer in.
2. Change our delegator contract to use `msg.data` directly, instead of re-encoding. This removes some unnecessary repacking.
3. Reduce potentially redunant SLOADs by memoizing values.
4. Do not re-accrue interest multiple-times per block (as the interest accrued is, by definition, zero, beyond the first transaction in a block for a given cToken).
5. Remove `localVars` structs as Solidity handles more variables correctly and it cleans up code.
6. In some cases, switch from `CarefulMath` to `SafeMath` (that is, revert instead of returning error codes). This is done in select places as it's technically a change of behavior and needs to be handled carefully.
7. Add Tether to Price Oracle Proxy.
8. Bump up to 0.5.16, but:
9. Update Solidity 0.6.x-style contract inheritence (that is, interfaces cannot declare constants, etc)