The render method now returns a promise. Once this promise is resolved,
the _position method for that node will be called (if applicable). This
promise must be resolved only after all subordinate nodes have completed
their render phase (the promise returned by subordinate node's render
method has resolved). Node that do not have subordinates can return the
result of calling terminalRender, and proxied renders only need to
return the result of calling proxy.
With this change, it is no longer necessary to explicitly position
subordinate nodes. They will already be positioned once their render
promise is resolved.
This is to eventually simplify the combining of Literals together (and
to actually make it work correctly, since terminals should not have been
merged)
