Add a simple wrapper for address. (#1773)

* Updated code style to no more than120 characters per line.

* Unify code comments style with Doxygen-style tags.

* Fix the conflicts.

* Add a return value in the contract ERC20Burnable.

* A Add a wrapper function to change type of address to address payable.

* U Modify Address utils.

* A Add test case for Address.

* U Modify code style in ERC20Burnable.

* Add changelog entry.

* Improved dev docs.

test/utils/Address.test.js

@@ -1,19 +1,37 @@
-require('openzeppelin-test-helpers');
+const { constants } = require('openzeppelin-test-helpers');
 
 const AddressImpl = artifacts.require('AddressImpl');
 const SimpleToken = artifacts.require('SimpleToken');
 
 contract('Address', function ([_, other]) {
+  const ALL_ONES_ADDRESS = '0xFFfFfFffFFfffFFfFFfFFFFFffFFFffffFfFFFfF';
+
   beforeEach(async function () {
     this.mock = await AddressImpl.new();
   });
 
-  it('should return false for account address', async function () {
-    (await this.mock.isContract(other)).should.equal(false);
+  describe('isContract', function () {
+    it('should return false for account address', async function () {
+      (await this.mock.isContract(other)).should.equal(false);
+    });
+
+    it('should return true for contract address', async function () {
+      const contract = await SimpleToken.new();
+      (await this.mock.isContract(contract.address)).should.equal(true);
+    });
   });
 
-  it('should return true for contract address', async function () {
-    const contract = await SimpleToken.new();
-    (await this.mock.isContract(contract.address)).should.equal(true);
+  describe('toPayable', function () {
+    it('should return a payable address when the account is the zero address', async function () {
+      (await this.mock.toPayable(constants.ZERO_ADDRESS)).should.equal(constants.ZERO_ADDRESS);
+    });
+
+    it('should return a payable address when the account is an arbitrary address', async function () {
+      (await this.mock.toPayable(other)).should.equal(other);
+    });
+
+    it('should return a payable address when the account is the all ones address', async function () {
+      (await this.mock.toPayable(ALL_ONES_ADDRESS)).should.equal(ALL_ONES_ADDRESS);
+    });
   });
 });