refactor(chain): compute txid once for `KeychainTxOutIndex::index_tx` #395

Job	Run time
Check no_std	39s
clippy_check	1m 40s
Check WASM	1m 18s
Build and test (stable, true, --no-default-features)	4m 9s
Build and test (stable, true, --all-features)	4m 14s
Rust fmt	4s
Build and test (1.63.0, --no-default-features)	7m 1s
Build and test (1.63.0, --all-features)	6m 54s
	25m 59s

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