Draft and lifecycles directories cleanup (#2122)

* Move Pausable into utils

* Move Strings into utils

* Move Counters into utils

* Move SignedSafeMath into math

* Remove ERC1046

* Make ERC20Snapshot.snapshot internal

* Move ERC20Snapshot into ERC20

* Add drafts deprecation notice

* Remove drafts directory

* Add changelog entry

* Apply suggestions from code review

Co-Authored-By: Francisco Giordano <frangio.1@gmail.com>

Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

test/utils/Strings.test.js

@@ -0,0 +1,26 @@
+const { contract } = require('@openzeppelin/test-environment');
+const { constants } = require('@openzeppelin/test-helpers');
+
+const { expect } = require('chai');
+
+const StringsMock = contract.fromArtifact('StringsMock');
+
+describe('Strings', function () {
+  beforeEach(async function () {
+    this.strings = await StringsMock.new();
+  });
+
+  describe('from uint256', function () {
+    it('converts 0', async function () {
+      expect(await this.strings.fromUint256(0)).to.equal('0');
+    });
+
+    it('converts a positive number', async function () {
+      expect(await this.strings.fromUint256(4132)).to.equal('4132');
+    });
+
+    it('converts MAX_UINT256', async function () {
+      expect(await this.strings.fromUint256(constants.MAX_UINT256)).to.equal(constants.MAX_UINT256.toString());
+    });
+  });
+});