Safe Casting Library from uint256 to uintXX (#1926)

* Include Safe Casting Library with complete and exhaustive test-suite.

* linting test file.

* Typo in SafeCast import statement

* Update test/utils/SafeCast.test.js

* Rename `castUXX` to `toUintXX` from suggestion

* Tackling the quick and easy suggestions regarding error string improvements etc.

* typo and changelog update.

* Improve SafeCast tests

* Update test/utils/SafeCast.test.js

* Update test/utils/SafeCast.test.js

* incorrect import

* add SafeCast to docs site

* Update CHANGELOG.md

* Update SafeCast.sol

test/utils/SafeCast.test.js

@@ -0,0 +1,45 @@
+const { BN, expectRevert } = require('@openzeppelin/test-helpers');
+
+const { expect } = require('chai');
+
+const SafeCastMock = artifacts.require('SafeCastMock');
+
+contract('SafeCast', async () => {
+  beforeEach(async function () {
+    this.safeCast = await SafeCastMock.new();
+  });
+
+  function testToUint (bits) {
+    describe(`toUint${bits}`, () => {
+      const maxValue = new BN('2').pow(new BN(bits)).subn(1);
+
+      it('downcasts 0', async function () {
+        expect(await this.safeCast[`toUint${bits}`](0)).to.be.bignumber.equal('0');
+      });
+
+      it('downcasts 1', async function () {
+        expect(await this.safeCast[`toUint${bits}`](1)).to.be.bignumber.equal('1');
+      });
+
+      it(`downcasts 2^${bits} - 1 (${maxValue})`, async function () {
+        expect(await this.safeCast[`toUint${bits}`](maxValue)).to.be.bignumber.equal(maxValue);
+      });
+
+      it(`reverts when downcasting 2^${bits} (${maxValue.addn(1)})`, async function () {
+        await expectRevert(
+          this.safeCast[`toUint${bits}`](maxValue.addn(1)),
+          `SafeCast: value doesn't fit in ${bits} bits`
+        );
+      });
+
+      it(`reverts when downcasting 2^${bits} + 1 (${maxValue.addn(2)})`, async function () {
+        await expectRevert(
+          this.safeCast[`toUint${bits}`](maxValue.addn(2)),
+          `SafeCast: value doesn't fit in ${bits} bits`
+        );
+      });
+    });
+  }
+
+  [8, 16, 32, 64, 128].forEach(bits => testToUint(bits));
+});