Simplify and fix generator tree management

This makes a number of related changes to the generator tree
management, that should hopefully make it easier to understand,
more robust and faster for the common linear-chain case. Fixes
https://bugs.php.net/bug.php?id=80240, which was the original
motivation here.

 * Generators now only add a ref to their direct parent.
 * Nodes only store their children, not their leafs, which avoids
   any need for leaf updating. This means it's no longer possible
   to fetch the child for a certain leaf, which is something we
   only needed in one place (update_current). If multi-children
   nodes are involved, this will require doing a walk in the other
   direction (from leaf to root). It does not affect the common
   case of single-child nodes.
 * The root/leaf pointers are now seen as a pair. One leaf generator
   can point to the current root. If a different leaf generator is
   used, we'll move the root pointer over to that one. Again, this
   is a cache to make the common linear chain case fast, trees may
   need to scan up the parent link.

Closes GH-6344.

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