Function depthFirst

Navigate from a node to the next node in a depth-first pre-order traversal, where parents are visited before their children, and both are visited before the next sibling of the parent. For example:

For the tree:
                  ┌─┐
                  │1│
             ╭────┴─┴─────╮
           ┌─┴─┐        ┌─┴─┐
           │1.1│        │1.2│
         ╭─┴───┴╮      ╭┴───┴─╮
      ┌──┴──┐┌──┴──┐┌──┴──┐┌──┴──┐
      │1.1.1││1.1.2││1.2.1││1.2.2│
      └─────┘└─────┘└─────┘└─────┘

     depthFirst ┊ result of
    called on...┊ navigation
    ┈┈┈┈┈┈┈┈┈┈┈┈┼┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈
           “1”  ┊ “1.1”
         “1.1”  ┊ “1.2”
         “1.2”  ┊ “1.1.1”
       “1.1.1”  ┊ “1.1.2”
       “1.2.1”  ┊ “1.2.2”
       “1.2.2”  ┊ Exception thrown
Copy

Returns Option.none when the final node in the traversal has been reached.

This is the unsafe version of tryDepthFirst.