Move the type constraint on the function, not the body

[davesnx]

On OCaml 4.14, ppxlib converts let f x = (x : int) to Pexp_function with the type constraint attached to the function itself (Some (Pconstraint int)) rather than as a Pexp_constraint wrapping the body.

The curriedPatternsAndReturnVal method returned the whole Pexp_function as the "return value" when a constraint was present, causing infinite recursion when the printer tried to process it again.

The fix was to extract the body expression and wrap it in Pexp_constraint/Pexp_coerce when the function has a constraint, so the printer can process it without recursion.