Ordnode and private definitions.

Stream: mathlib4

[Wrenna Robson]

I happened to just notice that in Ordnode (https://leanprover-community.github.io/mathlib4_docs/Mathlib/Data/Ordmap/Ordnode.html) there are a few definitions that are specifically called out as intended for internal use only (and in general I think this applies to many definitions for this type because there are some fundamental invariants that are not enforced at the type level). Putting aside the question as to whether that's a good idea, this seems like a great use for the private keyword and possible modules (I don't understand modules).

[Kim Morrison]

Just move to Cslib?