dirstate-tree: Add `NodeRef` and `ChildNodesRef` enums
They are used instead of `&Node` and `&ChildNodes` respectively.
The `ChildNodes` type alias also becomes a similar enum.
For now they only have one variant each, to be extended later.
Adding enums now forces various use sites go through new methods
instead of manipulating the underlying data structure directly.
Differential Revision: https://phab.mercurial-scm.org/D10747
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