Tether Support and Gas Optimizations

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)
10. Add a scenario command to check the BorrowRate from an IR model
11. Deploy Tether to Ropsten and Mainnet
12. Bump up total gas limit

contracts/CDaiDelegate.sol

@@ -181,38 +181,28 @@ contract CDaiDelegate is CErc20Delegate {
 
 /*** Maker Interfaces ***/
 
-contract PotLike {
-    function chi() public view returns (uint);
-    function dsr() public view returns (uint);
-    function rho() public view returns (uint);
-    function pie(address) public view returns (uint);
-    function drip() public returns (uint);
-    function join(uint) public;
-    function exit(uint) public;
+interface PotLike {
+    function chi() external view returns (uint);
+    function pie(address) external view returns (uint);
+    function drip() external returns (uint);
+    function join(uint) external;
+    function exit(uint) external;
 }
 
-contract GemLike {
-    function approve(address, uint) public;
-    function balanceOf(address) public view returns (uint);
-    function transfer(address, uint) public;
-    function transferFrom(address, address, uint) public;
-    function deposit() public payable;
-    function withdraw(uint) public;
+interface GemLike {
+    function approve(address, uint) external;
+    function balanceOf(address) external view returns (uint);
+    function transferFrom(address, address, uint) external returns (bool);
 }
 
-contract VatLike {
-    function can(address, address) public view returns (uint);
-    function ilks(bytes32) public view returns (uint, uint, uint, uint, uint);
-    function dai(address) public view returns (uint);
-    function urns(bytes32, address) public view returns (uint, uint);
-    function frob(bytes32, address, address, address, int, int) public;
-    function hope(address) public;
-    function move(address, address, uint) public;
+interface VatLike {
+    function dai(address) external view returns (uint);
+    function hope(address) external;
 }
 
-contract DaiJoinLike {
-    function vat() public returns (VatLike);
-    function dai() public returns (GemLike);
-    function join(address, uint) public payable;
-    function exit(address, uint) public;
+interface DaiJoinLike {
+    function vat() external returns (VatLike);
+    function dai() external returns (GemLike);
+    function join(address, uint) external payable;
+    function exit(address, uint) external;
 }

contracts/CErc20.sol

@@ -130,29 +130,10 @@ contract CErc20 is CToken, CErc20Interface {
         return token.balanceOf(address(this));
     }
 
-    /**
-     * @dev Checks whether or not there is sufficient allowance for this contract to move amount from `from` and
-     *      whether or not `from` has a balance of at least `amount`. Does NOT do a transfer.
-     */
-    function checkTransferIn(address from, uint amount) internal view returns (Error) {
-        EIP20Interface token = EIP20Interface(underlying);
-
-        if (token.allowance(from, address(this)) < amount) {
-            return Error.TOKEN_INSUFFICIENT_ALLOWANCE;
-        }
-
-        if (token.balanceOf(from) < amount) {
-            return Error.TOKEN_INSUFFICIENT_BALANCE;
-        }
-
-        return Error.NO_ERROR;
-    }
-
     /**
      * @dev Similar to EIP20 transfer, except it handles a False result from `transferFrom` and reverts in that case.
-     *      If caller has not called `checkTransferIn`, this may revert due to insufficient balance or insufficient
-     *      allowance. If caller has called `checkTransferIn` prior to this call, and it returned Error.NO_ERROR,
-     *      this should not revert in normal conditions. This function returns the actual amount received,
+     *      This will revert due to insufficient balance or insufficient allowance.
+     *      This function returns the actual amount received,
      *      which may be less than `amount` if there is a fee attached to the transfer.
      *
      *      Note: This wrapper safely handles non-standard ERC-20 tokens that do not return a value.