Rename TreePosition (and C structs) to TreeIndexes

[jeromekelleher]

In discussion with @benjeffery we decided that TreeIndexes is a better name for the low-level structure used to keep track of how we iterate over trees than TreePosition.

In an extension to this, we'll use DescendantIndexes as the class holding the information required to efficiently iterate over descendants in an graph type way and AncestorIndexes as the class for doing the bottom-to-top traversals (adapting from #2869)

[jeromekelleher]

+1 for doing this pre 1.0. Have we released the C API with a documented tree_pos struct?

[benjeffery]

No, it's still undocumented.

[jeromekelleher]

This is about the C API so dropping from the Python milestone