Enable paths to reduced node

[zspitz]

Some writer/visitors expand dynamic nodes by reducing them into a new expression tree. Currently, the path spans don't handle these reduced nodes.

Ideally, reduced nodes would be valid in the path spans, and Pather (which we're using to test the paths) should recognize the reduced node as having to call Reduce and continue on the generated tree. (see https://github.com/Domysee/Pather.CSharp#teaching-pathercsharp-new-tricks)

[zspitz]

This would be a good idea to optionally add to the textual tree renderer, perhaps with some indication that the resulting branch comes from reduced nodes. There's no need for this; the node itself could have a child node "Reduce()", the children of which would be the result of calling Reduce.

We could then generate the paths from the textual tree renderer and use that as a reference point to test the paths for other renderers.