use PlaceRef more consistently instead of loosely coupled local+projection
Instead of working directly with the `projections` array, use `iter_projections` and `last_projection`. This avoids having to construct new `PlaceRef` from the pieces everywhere.
I only did this for a few files, to see how people think about this. If y'all are happy with this, I'll open an E-mentor issue to complete this. I grepped for `Place::ty_from` to find the places that need adjusting -- this could miss some, but I am not sure what else to grep for.
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