Add an extra factory function for 'SetOfMergedPaths' that doesn't use a filter.
Minor refactoring - name some previously anonymous code blocks in the implementation of 'SetOfMergedPaths' for clarity.
Replace 'Node.FoldRightPostOrder' with 'Node.FoldLeftPostOrder'.
Merge in replacement of 'FoldRightPostOrder' by 'FoldLeftPostOrder'.
Add 'Tag' property to 'Node'.
WIP commit: changed the tag API after thinking about how messy it would be to implement a dual dictionary / tagged inputs interface on a filter inout. Now using two overloads of 'WithTag' on factories and nodes.