Remove unnecessary top-level types and code.

This code clean-up removes unnecessary container types from gorax.  This
saves a fair bit of redundant typing, without breaking backward
compatibility.  Thanks to smw on #go-nuts IRC channel for the
suggestion.

This change was instructional for another reason -- I was going to add
support for server actions, which is 7-ish more endpoints, all of which
had their own JSON containers.  That proliferation of FooContainer types
would have crossed the line towards unmaintainable code, especially
considering the number of duplicate-named containers which have
different inner types.