Add EIP 712 helpers (#2418)

contracts/mocks/EIP712External.sol

@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity >=0.6.0 <0.8.0;
+
+import "../drafts/EIP712.sol";
+import "../cryptography/ECDSA.sol";
+
+contract EIP712External is EIP712 {
+    constructor(string memory name, string memory version) public EIP712(name, version) {}
+
+    function domainSeparator() external view returns (bytes32) {
+        return _domainSeparatorV4();
+    }
+
+    function verify(bytes memory signature, address signer, address mailTo, string memory mailContents) external view {
+        bytes32 digest = _hashTypedDataV4(keccak256(abi.encode(
+            keccak256("Mail(address to,string contents)"),
+            mailTo,
+            keccak256(bytes(mailContents))
+        )));
+        address recoveredSigner = ECDSA.recover(digest, signature);
+        require(recoveredSigner == signer);
+    }
+
+    function getChainId() external pure returns (uint256 chainId) {
+        // solhint-disable-next-line no-inline-assembly
+        assembly {
+            chainId := chainid()
+        }
+    }
+}