feat(chain)!: introduce new `TopologicalIterWithLevels` #250

Job	Run time
prepare	3s
Rust fmt	16s
Check no_std	33s
Build & Test (1.88.0, true, --all-features)	2m 2s
Build & Test (1.75.0, --all-features)	2m 3s
Build & Test (1.88.0, true, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	1m 45s
Build & Test (1.63.0, --all-features)	2m 2s
Build & Test (1.63.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	2m 2s
Build & Test Examples (example_cli)	40s
Build & Test Examples (example_esplora)	57s
Build & Test (1.75.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	1m 53s
Build & Test Examples (example_bitcoind_rpc_polling)	46s
Rust clippy	1m 12s
Build & Test Examples (example_electrum)	1m 1s
Check WASM	2m 12s
	19m 27s

Provide feedback