Fix let-bindings with pattern constraints
Fix let-bindings with pattern constraints

Formatting a let-binding with a constraint pattern results in moving
the constraint to the body expression. Strengthen normalization to
avoid asserting on e.g.:
```
let (x : int) = 0
```

ocaml-ppx/ocamlformat,ocaml-ppx/ocamlformat,ocaml-ppx/ocamlformat