Remove Ppxlib's label exposed

[davesnx]

There seems to be a type "label" assigned to string which messes a bit with error messages and inference when you use ppxlib with open Ppxlib. Is there any chance to fix it?

I endup doing a workaround like (unsure if there's a better way)

type s = string
open Ppxlib
type string = s

[NathanReb]

I agree this is annoying but it comes from the Asttypes that are in scope when opening Ppxlib. We obviously can't remove them from the scope as it would be a huge breaking change.

I'll try to see if we can incorporate a similar trick to yours directly into ppxlib.