feat(chain)!: introduce new `TopologicalIterWithLevels` #251

Job	Run time
prepare	5s
Rust fmt	14s
Check no_std	46s
Check WASM	1m 19s
Build & Test (1.88.0, true, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	2m 8s
Build & Test (1.63.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	4m 32s
Build & Test (1.75.0, --all-features)	3m 58s
Rust clippy	52s
Build & Test (1.63.0, --all-features)	5m 33s
Build & Test (1.75.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	2m 14s
Build & Test Examples (example_cli)	32s
Build & Test (1.88.0, true, --all-features)	3m 40s
Build & Test Examples (example_esplora)	51s
Build & Test Examples (example_bitcoind_rpc_polling)	38s
Build & Test Examples (example_electrum)	56s
	28m 18s

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