Refactor TreeArena #2683

Job	Run time
cargo clippy (wasm32)	1m 59s
formatting	32s
cargo check (msrv) (wasm32)	17s
cargo check (msrv) (windows-latest)	2m 0s
cargo clippy (windows-latest)	4m 54s
cargo check (msrv) (macos-latest)	46s
cargo clippy (macos-latest)	3m 8s
cargo miri (unsafe tree_arena)	32s
cargo check (aarch64-android)	21s
cargo check (msrv) (ubuntu-latest)	42s
cargo clippy (ubuntu-latest)	3m 15s
Prime LFS Cache	4s
cargo test (wasm32)	19s
cargo doc	41s
typos	7s
cargo test (windows-latest)	3m 4s
cargo test (macos-latest)	2m 1s
cargo test (ubuntu-latest)	3m 53s
	28m 35s

Provide feedback