In theory argument elaboration order doesn't matter, but in practice we
sometimes make choices for performance reasons, like helping with
disambiguation by knowing the target type.
This was kind of messy, now we can more clearly see what's going on.
Also, more importantly, it gives us a bit more control which we
sometimes need. For example, if we go choose to go right to left for
some performance heuristic it might turn out we don't have enough
information yet, in which case we need to delay and try again later.
Fixes#1743
