Use Hashmap to track TreeArena node's children (#772) #2373

Job	Run time
cargo check (msrv) (wasm32)	20s
cargo check (aarch64-android)	26s
cargo clippy (windows-latest)	5m 17s
cargo check (msrv) (windows-latest)	1m 55s
cargo clippy (macos-latest)	2m 33s
cargo check (msrv) (macos-latest)	46s
cargo clippy (wasm32)	2m 8s
cargo clippy (ubuntu-latest)	3m 19s
cargo doc	44s
cargo check (msrv) (ubuntu-latest)	40s
formatting	28s
Prime LFS Cache	5s
cargo test (wasm32)	22s
typos	5s
cargo test (windows-latest)	4m 29s
cargo test (macos-latest)	1m 37s
cargo test (ubuntu-latest)	4m 39s
	29m 53s

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