Zipper exercise instructions don't match the implementation stub

[stewartmurrie]

The instructions for the Zipper exercise discuss type structures and functions which don't align with the stub student's are asked to complete. Specifically:

• The instructions mention a rose tree structure where nodes contain lists of children, whereas the type defined in the stub is a simple binary tree
• The instructions include functions insert_before, insert_after, prev, next, and delete, none of which are defined in the stub
• The stub includes functions set_left and set_right, whose behaviour isn't defined in the instructions.

The student experience would be better if the two lined up.