Allow to generate a random sring

Summary:
These functions are used in Zed.

This is a new implementation, using closures,
to decrease cyclomatic complexity, and pure
random functions to generate digits or letters
instead to play with arrays of elements.

Test Plan: Unit tests included

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1629