switch from tree to index tree (arena)

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tree has many tradeoffs, especially

• cycle will break function
• many lock/unlock operation is necessary in rust
• makes it hard to do multi-processing

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Ref:

• https://docs.rs/indextree/4.2.0/indextree/
• https://github.com/fitzgen/generational-arena

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This is urgent to have, I'll try to do it

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To implement this, saschagrunert/indextree#53 is necessary