feat: add rlp length helper (#1906) #5733

Job	Run time
docs	2m 5s
clippy	1m 42s
doctest	3m 2s
feature-checks	8m 36s
fmt	12s
check no_std	57s
test ubuntu-latest stable --no-default-features	2m 40s
wasm-unknown	2m 45s
test ubuntu-latest stable	2m 33s
wasm-wasi	3m 57s
test ubuntu-latest stable --all-features	3m 12s
test ubuntu-latest nightly --no-default-features	2m 48s
test ubuntu-latest nightly	2m 36s
test ubuntu-latest nightly --all-features	2m 57s
test ubuntu-latest 1.81 --no-default-features	1m 46s
test ubuntu-latest 1.81	1m 48s
test windows-latest stable --no-default-features	6m 50s
test windows-latest stable	6m 12s
test windows-latest stable --all-features	7m 35s
test windows-latest nightly --no-default-features	6m 36s
test windows-latest nightly	6m 45s
test windows-latest nightly --all-features	7m 42s
test windows-latest 1.81 --no-default-features	5m 1s
test windows-latest 1.81	4m 23s
deny / cargo deny check	20s
ci success	4s
	1h 35m 4s

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