feat: moved enum and events to USM.sol

contracts/IUSM.sol

@@ -3,12 +3,6 @@ pragma solidity ^0.6.7;
 
 
 interface IUSM {
-    enum Side {Buy, Sell}
-
-    event MinFumBuyPriceChanged(uint previous, uint latest);
-    event MintBurnAdjustmentChanged(uint previous, uint latest);
-    event FundDefundAdjustmentChanged(uint previous, uint latest);
-
     function mint(address from, address to, uint ethIn) external returns (uint);
     function burn(address from, address to, uint usmToBurn) external returns (uint);
     function fund(address from, address to, uint ethIn) external returns (uint);

contracts/USM.sol

@@ -20,6 +20,12 @@ contract USM is IUSM, ERC20Permit, Delegable {
     using SafeMath for uint;
     using WadMath for uint;
 
+    enum Side {Buy, Sell}
+
+    event MinFumBuyPriceChanged(uint previous, uint latest);
+    event MintBurnAdjustmentChanged(uint previous, uint latest);
+    event FundDefundAdjustmentChanged(uint previous, uint latest);
+
     uint public constant WAD = 10 ** 18;
     uint public constant MAX_DEBT_RATIO = WAD * 8 / 10;                 // 80%
     uint public constant MIN_FUM_BUY_PRICE_HALF_LIFE = 24 * 60 * 60;    // 1 day

contracts/WadMath.sol

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
 pragma solidity ^0.6.7;
 
 import "@openzeppelin/contracts/math/SafeMath.sol";