A function which returns the absolute value of a signed value (#2984)

* Add a function which returns the absolute (and obviously unsigned) value of a signed value.

* add changelog entry and fix lint

Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>

test/utils/math/Math.test.js

@@ -1,6 +1,6 @@
 const { BN, constants } = require('@openzeppelin/test-helpers');
 const { expect } = require('chai');
-const { MAX_UINT256 } = constants;
+const { MAX_UINT256, MAX_INT256, MIN_INT256 } = constants;
 
 const MathMock = artifacts.require('MathMock');
 
@@ -85,4 +85,20 @@ contract('Math', function (accounts) {
       expect(await this.math.ceilDiv(MAX_UINT256, b)).to.be.bignumber.equal(MAX_UINT256);
     });
   });
+
+  describe('abs', function () {
+    for (const n of [
+      MIN_INT256,
+      MIN_INT256.addn(1),
+      new BN('-1'),
+      new BN('0'),
+      new BN('1'),
+      MAX_INT256.subn(1),
+      MAX_INT256,
+    ]) {
+      it(`correctly computes the absolute value of ${n}`, async function () {
+        expect(await this.math.abs(n)).to.be.bignumber.equal(n.abs());
+      });
+    }
+  });
 });