Checks a corner case with type checking involving destructuring binds.

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

``` ucm :hide
scratch/main> builtins.merge
```

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


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