Feature Question: Record Subtyping

We have been playing around with using Dex record types for generic name-based contraction. The lens / zipper example is clear about how make this work.

It does immediately make me wonder though if you can do broadcasting / subtyping on records. In particular given two record index types it would be nice to write a function like this:

```haskell
def promote (tab: {...b} => out) : {a & ...b} => out =
for i : {a & ...b}.
tab.(down i)
```
I thought a bit about how to do it, but was completely stumped. I realize that you can do it by providing the lens explicitly, but that is less satisfying.

```haskell
-- Probably an easier way to do this...
def push (name: Iso a (b & c))
(tensor1: b => c => d ) : a => d =
for i : a.
index1 = getAt name i
index2 = popAt name i
tensor1.index1.index2

def ndim (name: Iso a (b & c))
(tensor: c => out) : (a => out) =
push name for i: b. tensor
```

I see that this does exist in http://hackage.haskell.org/package/generic-lens-2.0.0.0/docs/Data-Generics-Product-Subtype.html , but perhaps it requires some trickery.

Seems related to https://github.com/google-research/dex-lang/issues/258

Start with the lens/zipper example described in the issue and review the related discussion in issue #258. Compare the requested record broadcasting and subtyping behavior with Data.Generics.Product.Subtype and determine whether Dex needs a language or type-system change. Done means reaching a clear feasibility decision and documenting the required scope.

Written by the indexing model from the issue text.