Add Base64 library to utils (#2884)

* Add Base64 library to utils

* Fix typo on Base64 padding

* Added documentation for Base64 and references from ERC1155 and ERC721

* Updated Changelog

* Fix typo in utilities doc

* use mstore8 to improve memory accesses

* use shorter strings with encodePacked

* do not use using-for syntax, for clarity

Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>
Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

test/utils/Base64.test.js

@@ -0,0 +1,29 @@
+const { expect } = require('chai');
+
+const Base64Mock = artifacts.require('Base64Mock');
+
+contract('Strings', function () {
+  beforeEach(async function () {
+    this.base64 = await Base64Mock.new();
+  });
+
+  describe('from bytes - base64', function () {
+    it('converts to base64 encoded string with double padding', async function () {
+      const TEST_MESSAGE = 'test';
+      const input = web3.utils.asciiToHex(TEST_MESSAGE);
+      expect(await this.base64.encode(input)).to.equal('dGVzdA==');
+    });
+
+    it('converts to base64 encoded string with single padding', async function () {
+      const TEST_MESSAGE = 'test1';
+      const input = web3.utils.asciiToHex(TEST_MESSAGE);
+      expect(await this.base64.encode(input)).to.equal('dGVzdDE=');
+    });
+
+    it('converts to base64 encoded string without padding', async function () {
+      const TEST_MESSAGE = 'test12';
+      const input = web3.utils.asciiToHex(TEST_MESSAGE);
+      expect(await this.base64.encode(input)).to.equal('dGVzdDEy');
+    });
+  });
+});