add exercise circular buffer

Test program is pretty close to the Ruby and Javascript programs, but it tests a buffer of bytes rather than arbitrary types.  A fixed type seemed more Go-like.  Given this, the rule of ignoring nil makes no sense so I skipped that.

The example uses the container/ring type from the standard library.  This has some inefficiencies compared a slice-based algorithm like in the Ruby or JS examples or like others described on WP, but it's slightly easier.