Challenge for L while

[jsiek]

Add documentation for the L_while "constant propagation" challenge
- I think some people split this pass into 3 different processes, called "constant folding", "constant propagation", and "dead-code elimination"? I don't think that dead-code elimination should be documented more than a "hey it'd be cool", but falling in line with the nano-pass philosophy by splitting the pass into two could be useful