Bring `cheatsheet` into `master` branch as directory

[jtmoulia]

Problem

Currently the cheatsheet is a separate branch in this repo. This means that commits to the master branch which change tests require similar changes to the cheatsheet branch.

Solution

lisp-koans uses a directory in the master branch (koan-solutions) to store the solutions. A similar solution should be implemented where the solutions in the cheatsheet branch are brought into a directory under the master branch. I kinda like the name cheatsheet for the directory, but that's minor.

A mechanism should be added for loading the cheatsheet as opposed to the current koans so they can be easily tested. This should be documented in the README.