*     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.