Fix breed methods

See https://github.com/NetLogo/NetLogo-Headless/issues/3. I have a number of commits here because I wanted to show the development of the refactor. They are all squashable upon request. The StructureChecker refactor (77494d1) is only partially complete. Depending on how we want to go with the refactor, I could:
 1) remove it entirely
 2) pull it over to another branch
 3) finish it on this branch before merging in
Very unusual to be defining your own destructuring code.  Why not just declare `Helper` as a case class?  If you want to don't care about the `program` argument, you can either have just match it with `_`, or you can put it in a second parameter list in the case class's constructor.