New exercise type: math / programming

[heatherleaf]

There is already an example of a simple imperative programming language, but there is no good exercise for it. Here are some ideas of how to do this:

• the sentence to edit is a math formula or a program
• the goal is that the formula/program respects some property
• the system shows counterexamples as long as its wrong
  • we can use QuickCheck to find good counterexamples

Examples:

• implement a sorting function -- the counterexamples can be pairs of input-output lists (where we see that the output list is not sorted)
• implement a program that draws a given picture -- this is similar to New exercise type: picture description #91

[heatherleaf]

Depends on #99