add skeleton Q104.104 implementation

contracts/libraries/MOCK_Decimal.sol

@@ -1,20 +0,0 @@
-pragma solidity 0.5.12;
-
-// mock fixed- or floating-point math
-library MOCK_Decimal {
-    struct Decimal {
-        uint256 data;
-    }
-
-    function div(uint128 numerator, uint128 denominator) internal pure returns (Decimal memory) {
-        return Decimal(numerator / denominator);
-    }
-
-    function mul(Decimal memory a, uint128 b) internal pure returns (Decimal memory) {
-        return Decimal(a.data * b);
-    }
-
-    function add(Decimal memory a, Decimal memory b) internal pure returns (Decimal memory) {
-        return Decimal(a.data + b.data);
-    }
-}

contracts/libraries/SafeMath256.sol

@@ -15,4 +15,9 @@ library SafeMath256 {
         require(y <= uint128(-1), "downcast-128-overflow");
         z = uint128(y);
     }
+
+    function downcast32(uint256 y) internal pure returns (uint32 z) {
+        require(y <= uint32(-1), "downcast-32-overflow");
+        z = uint32(y);
+    }
 }

contracts/libraries/UQ104x104.sol

@@ -0,0 +1,17 @@
+pragma solidity 0.5.12;
+
+// TODO this whole library is basically just a mock at the moment
+library UQ104x104 {
+    uint208 constant Q104 = uint104(-1);
+
+    function encode(uint128 y) internal pure returns (uint208 z) {
+        // require(y <= Q104, "encode-overflow");
+        z = y * Q104;
+    }
+    function qmul(uint208 x, uint208 y) internal pure returns (uint208 z) {
+        z = x * y / Q104;
+    }
+    function qdiv(uint208 x, uint208 y) internal pure returns (uint208 z) {
+        z = x / y;
+    }
+}