Simplify and fix generator tree management

[nikic]

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.

[nikic]

Note that the output here is incorrect (only one next() call should be needed above). However, this is a pre-existing bug in PHP 7.4 and not related to this change.

[bwoebi]

I like it. Looks good to me and I cannot think right now of a scenario where this would break.
Feel free to merge :-)