Implement a convenience recursive walk method over a cursor and its descendants.

Before r160106 there was a way to recursively visit all descendants of a cursor
via Cursor_visit, but it was removed. Since then, every user needs to
reimplement the recursive descent into get_children.

Adding a walk_preorder() method to Cursor that conveniently implements recursive
walking in a Pythonic way. This also greatly simplifies get_cursor and
get_cursors in tests/cindex/util.py (walk_preorder is now tested through these
utility functions, since they are used in many tests).