Add Arrays library with unit tests (#1209) (#1375)

*     Add Arrays library with unit tests (#1209)

    * prepared due to snapshot token requirements
    * add library with method to find upper bound
    * add unit test for basic and edge cases

* Imporove documentation for Arrays library

Simplify Arrays.test.js to use short arrays as test date

* Added comment for uint256 mid variable.
* Explaned why uint256 mid variable calculated as Math.average is safe to use as index of array.

(cherry picked from commit f7e53d90fa)

contracts/mocks/ArraysImpl.sol

@@ -0,0 +1,18 @@
+pragma solidity ^0.4.24;
+
+import "../utils/Arrays.sol";
+
+contract ArraysImpl {
+
+  using Arrays for uint256[];
+
+  uint256[] private array;
+
+  constructor(uint256[] _array) public {
+    array = _array;
+  }
+
+  function findUpperBound(uint256 _element) external view returns (uint256) {
+    return array.findUpperBound(_element);
+  }
+}