Refactor TreeArena #2611

Job	Run time
cargo check (msrv) (wasm32)	18s
cargo check (aarch64-android)	19s
cargo check (msrv) (windows-latest)	1m 40s
cargo clippy (windows-latest)	4m 33s
cargo clippy (wasm32)	2m 3s
cargo check (msrv) (macos-latest)	43s
cargo clippy (macos-latest)	2m 10s
cargo check (msrv) (ubuntu-latest)	39s
cargo doc	40s
cargo clippy (ubuntu-latest)	3m 10s
formatting	33s
Prime LFS Cache	4s
cargo test (wasm32)	19s
typos	45s
cargo test (windows-latest)	4m 14s
cargo test (macos-latest)	1m 44s
cargo test (ubuntu-latest)	3m 38s
	27m 32s

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