Update unions

[JordonPhillips]

This updates generated unions. Now there will only be a single parent interface, and each member will only be a struct that implements that interface.

This also adds generation of unions to the protocol test generator.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[jasdel]

Not specific to this PR, but at some point we should take a look at how slice/map types are generated from $T, and $P. If a member is or is not a pointer of a slice/map should not be driven by $T, or $P, the member being a pointer is driven by the modeled list/map shape, and its member.

This mean that $T should generate the correct Go type for a map of strings, i.e. map[string]*string. In addition the same for lists (assuming lists cannot be sparse, i.e. []string. Using $P should generate the pointer type of the outer shape, *map[string]*string and *[]string. $P should have no impact on the generate of nested members within collections.

[jasdel]

created, #149 for this.