The AV::Base constructor was too complicated, and this commit tightens
up the parameters it will take.  At runtime, AV::Base is most commonly
constructed here:

  https://github.com/rails/rails/blob/94d54fa4ab641a0ddeb173409cb41cc5becc02a9/actionview/lib/action_view/rendering.rb#L72-L74

This provides an AV::Renderer instance, a hash of assignments, and a
controller instance.  Since this is the common case for construction, we
should remove logic from the constructor that handles other cases.  This
commit introduces special constructors for those other cases.
Interestingly, most code paths that construct AV::Base "strangely" are
tests.