Add Proxies from OpenZeppelin SDK (#2335)

contracts/mocks/RegressionImplementation.sol

@@ -0,0 +1,66 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+import "../proxy/Initializable.sol";
+
+contract Implementation1 is Initializable {
+  uint internal _value;
+
+  function initialize() public initializer {
+  }
+
+  function setValue(uint _number) public {
+    _value = _number;
+  }
+}
+
+contract Implementation2 is Initializable {
+  uint internal _value;
+
+  function initialize() public initializer {
+  }
+
+  function setValue(uint _number) public {
+    _value = _number;
+  }
+
+  function getValue() public view returns (uint) {
+    return _value;
+  }
+}
+
+contract Implementation3 is Initializable {
+  uint internal _value;
+
+  function initialize() public initializer {
+  }
+
+  function setValue(uint _number) public {
+    _value = _number;
+  }
+
+  function getValue(uint _number) public view returns (uint) {
+    return _value + _number;
+  }
+}
+
+contract Implementation4 is Initializable {
+  uint internal _value;
+
+  function initialize() public initializer {
+  }
+
+  function setValue(uint _number) public {
+    _value = _number;
+  }
+
+  function getValue() public view returns (uint) {
+    return _value;
+  }
+
+  // solhint-disable-next-line payable-fallback
+  fallback() external {
+    _value = 1;
+  }
+}