Add tsk_tree_position_step function to C API

[duncanMR]

It came up in #2651 (comment here) would be useful to have a function tsk_tree_position_step(&tree_pos, direction) in the C API which shifts the tree position forwards or backwards depending on the direction argument. This would be building on the modular incremental trees code introduced in #2786.

I implemented a Python version of this function in #2819; we need to translate it into C and add requisite tests and documentation.