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.

Author: nikic

Zend/tests/generators/backtrace_multi_yield_from.phpt

@@ -0,0 +1,32 @@
+--TEST--
+Generator backtrace with multi yield from
+--FILE--
+<?php
+
+function gen() {
+    yield 1;
+    debug_print_backtrace();
+    yield 2;
+}
+
+function from($gen) {
+    yield from $gen;
+}
+
+$gen1 = gen();
+$gen2 = from($gen1);
+$gen3 = from($gen2);
+var_dump($gen3->current());
+$gen2->next();
+var_dump($gen2->current());
+$gen2->next();
+var_dump($gen2->current());
+
+?>
+--EXPECTF--
+int(1)
+int(1)
+#0  gen() called at [%s:10]
+#1  from(Generator Object ())
+#2  Generator->next() called at [%s:19]
+int(2)

Zend/tests/generators/bug80240.phpt

@@ -0,0 +1,28 @@
+--TEST--
+Bug #80240: Use after free multi yield from
+--FILE--
+<?php
+
+function gen() {
+    yield 0;
+    yield from gen();
+}
+
+function bar($gen) {
+    yield from $gen;
+}
+
+$gen = gen();
+$a = bar($gen);
+$b = bar($gen);
+$a->rewind();
+$b->rewind();
+$a->next();
+unset($gen);
+unset($a);
+unset($b);
+
+?>
+===DONE===
+--EXPECT--
+===DONE===

Zend/tests/generators/yield_from_chain_dtor_order.phpt

@@ -0,0 +1,24 @@
+--TEST--
+Leaf link may need to be invalidated depending on dtor order
+--FILE--
+<?php
+
+function gen2() {
+    yield 1;
+}
+function gen() {
+    yield from gen2();
+}
+function bar($g) {
+    yield from $g;
+}
+
+$gen = gen();
+$bar = bar($gen);
+var_dump($bar->current());
+$copy = $bar;
+unset($gen);
+
+?>
+--EXPECT--
+int(1)