Handling tree data structure

Слиты nikitronn запросил слияние tree_walk_utils в master

Created by: Durman

Addressed problem description

Add helper code for handling node trees #3319 (closed)

Preflight checklist

  • dfs_walk
  • bfs_walk
  • toposort_walk
  • storing node indexes in tree structure
  • catching walk passes for optimisation stage _
  • Code changes complete.
  • Code documentation complete.
  • Unit-tests implemented.
  • Ready for merge.