Count ETupProject projlen differently / replace with simpler tuple indexing

We are using surface tuples throughout accelerate-backend kit, including decoding the `EltRepr` to recover the surface structure (which doesn't help us with shapes, alas).

However, there's an inconsistency here, because we count `projlen` in `ETupProject` in terms of scalar _leaves_. If we are sticking with surface, N-array tuples, then this should be replaced with a basic "project field N of M" operator, and this should be able to happen from when we first build `SimpleAcc`.