feat: add rlp length helper #5732

Job	Run time
wasm-unknown	2m 58s
test ubuntu-latest stable --no-default-features	2m 32s
check no_std	1m 0s
test ubuntu-latest stable	2m 31s
clippy	1m 30s
test ubuntu-latest stable --all-features	3m 14s
feature-checks	8m 32s
test ubuntu-latest nightly --no-default-features	2m 36s
doctest	3m 11s
fmt	12s
test ubuntu-latest nightly	2m 37s
wasm-wasi	3m 54s
test ubuntu-latest nightly --all-features	2m 54s
docs	1m 59s
test ubuntu-latest 1.81 --no-default-features	1m 39s
test ubuntu-latest 1.81	1m 37s
test windows-latest stable --no-default-features	6m 43s
test windows-latest stable	5m 33s
test windows-latest stable --all-features	6m 22s
test windows-latest nightly --no-default-features	6m 2s
test windows-latest nightly	6m 35s
test windows-latest nightly --all-features	6m 19s
test windows-latest 1.81 --no-default-features	3m 27s
test windows-latest 1.81	4m 22s
deny / cargo deny check	25s
ci success	2s
	1h 28m 46s

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