Use Hashmap to track TreeArena node's children #2370

Job	Run time
cargo check (aarch64-android)	28s
cargo clippy (windows-latest)	4m 50s
cargo check (msrv) (windows-latest)	1m 44s
cargo clippy (macos-latest)	2m 2s
cargo check (msrv) (macos-latest)	36s
cargo check (msrv) (wasm32)	19s
cargo clippy (ubuntu-latest)	3m 19s
cargo check (msrv) (ubuntu-latest)	42s
cargo clippy (wasm32)	1m 58s
cargo doc	46s
formatting	29s
Prime LFS Cache	7s
cargo test (wasm32)	21s
typos	4s
cargo test (windows-latest)	4m 36s
cargo test (macos-latest)	1m 32s
cargo test (ubuntu-latest)	4m 16s
	28m 9s

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