add exercise kindergarten-garden

On the surface this exercise is just transforming one data structure into another.  The devil's in the detail on this one though.  I ranked it nearly the most difficult exercise so far.

API choice was to let solver define a type, a constructor for it, and a method.  The method has the same signature as map index, but I left it the solver's choice for whether to actually use a map or not.  I used a map in example.go but I think there are excellent solutions without using a map.