* Adding and removing child nodes is now constant time, speed up should be huge.
* Searching for node paths as in ("path/to/node") should be far faster too.
This changes the children management and makes it a hashmap, optimizing most StringName based operations.
Most operations should be severe speed up without breaking compatibility.
This should fix many issues regarding to node access performance, and may also speed up editor start/end, but benchmarks are needed. So if you want to test, please make some benchmarks!
Further performance improvements will be done in the future by removing NOTIFICATION_MOVED_IN_PARENT and replacing by something less laborious.
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