add exercise saddle-points

Trying something new with building on the matrix task.  For some this might introduce the concept that a Go package can have multiple source files and that a single type can have methods defined in different source files.
Mm, well the function could certainly be written in different ways, but just replacing that continue with a break doesn't quite work.  Actually I was thinking of just specifying that Saddle must return results in lexicographic order.  Then a simple reflect.DeepEqual does the job.  I think I made the example produce lexicographic order.  I didn't test it though.