Checks a corner case with type checking involving destructuring binds.

The binds were causing some sequences of lets to be unnecessarily
recursive.

``` unison
unique ability Nate where
  nate: (Boolean, Nat)
  antiNate: ()


sneezy: (Nat -> {d} a) -> '{Nate,d} a
sneezy dee _ =
  (_,_) = nate
  antiNate
  dee 1
```

``` ucm

  Loading changes detected in scratch.u.

  I found and typechecked these definitions in scratch.u. If you
  do an `add` or `update`, here's how your codebase would
  change:
  
    ⍟ These new definitions are ok to `add`:
    
      ability Nate
      sneezy : (Nat ->{d} a) -> '{d, Nate} a

```