add exercise strain

Interesting choices with Go lacking abstraction features for generic collection code.  I went for the basics: slices, and following the Ruby tests, slices of ints, strings, and []int. Building abstractions with interfaces or with reflection seemed well outside the exercise, as did other possible things that could be used as collections, maps, container types from the standard library, and so on.

After many other exercises haven't really seemed to call for methods, I thought this exercise was a good application for named types with methods.  I didn't require a whole matrix of methods though, just

         | Int | []Int | String
-------|------|-------|--------
Keep |  X   |  X    |  X
Discard | X