Prove the eta rule for sigmas from a more general prinicple

Also, rename it to half_eta, because it says
```
  match u as u' return P u' with
    | (a; b) => (a; b)
  end
  ==
  u
```
rather than
```
  (match u as u' return A with
     | (a; b) => a
   end;
   match u as u' return P (match u' with (a; b) => a end) with
     | (a; b) => b
   end)
  ==
  u
```
which is the proper eta rule.  (More concisely, this is `(u.1; u.2) == u`.)