ocaml-json: hide Json_base as a private module

Json_base exposed low-level implementation tools (Rarray, Rbigarray1,
Type.Id polyfill, a mini Fmt, Number range predicates, a duplicate of
Json.Sort) even though nothing outside json.ml referenced it. Mark it
(private_modules json_base) so it stays a compile-time helper without
leaking into the public surface.

The user-facing re-export module Sort = Json_base.Sort in json.ml
keeps working: users see Json.Sort as documented in json.mli.