Add ERC20 _setTokenURI (#1618)

* Add _setTokenURI internal.

* Rename TokenMetadata to ERC20Metadata.

* Add changelog entry for ERC20Metadata.

* Fix linter error.

* Add breaking change changelog notice.

contracts/drafts/ERC1046/ERC20Metadata.sol

@@ -7,18 +7,18 @@ import "../../token/ERC20/IERC20.sol";
  * @dev See https://eips.ethereum.org/EIPS/eip-1046
  * @dev tokenURI must respond with a URI that implements https://eips.ethereum.org/EIPS/eip-1047
  */
-contract ERC20TokenMetadata is IERC20 {
-    function tokenURI() external view returns (string memory);
-}
-
-contract ERC20WithMetadata is ERC20TokenMetadata {
+contract ERC20Metadata {
     string private _tokenURI;
 
-    constructor (string memory tokenURI) public {
-        _tokenURI = tokenURI;
+    constructor (string memory tokenURI_) public {
+        _setTokenURI(tokenURI_);
     }
 
     function tokenURI() external view returns (string memory) {
         return _tokenURI;
     }
+
+    function _setTokenURI(string memory tokenURI_) internal {
+        _tokenURI = tokenURI_;
+    }
 }

contracts/mocks/ERC20MetadataMock.sol

@@ -0,0 +1,14 @@
+pragma solidity ^0.5.2;
+
+import "../token/ERC20/ERC20.sol";
+import "../drafts/ERC1046/ERC20Metadata.sol";
+
+contract ERC20MetadataMock is ERC20, ERC20Metadata {
+    constructor (string memory tokenURI) public ERC20Metadata(tokenURI) {
+        // solhint-disable-previous-line no-empty-blocks
+    }
+
+    function setTokenURI(string memory tokenURI) public {
+        _setTokenURI(tokenURI);
+    }
+}

contracts/mocks/ERC20WithMetadataMock.sol

@@ -1,10 +0,0 @@
-pragma solidity ^0.5.2;
-
-import "../token/ERC20/ERC20.sol";
-import "../drafts/ERC1046/TokenMetadata.sol";
-
-contract ERC20WithMetadataMock is ERC20, ERC20WithMetadata {
-    constructor (string memory tokenURI) public ERC20WithMetadata(tokenURI) {
-        // solhint-disable-previous-line no-empty-blocks
-    }
-}