Improvements to `PartialMmr::apply_delta()` #844

Job	Run time
Clippy with --features default,std,serde	25s
rustfmt	10s
Build stable on ubuntu with --no-default-features --target wasm32-unknown-unknown	17s
Clippy with --no-default-features	25s
Test stable on ubuntu with --features default,std,serde	44s
Build nightly on ubuntu with --no-default-features --target wasm32-unknown-unknown	22s
Test stable on ubuntu with --no-default-features	28s
Test nightly on ubuntu with --features default,std,serde	1m 53s
Test nightly on ubuntu with --no-default-features	39s
	5m 23s

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