feat: add rlp length helper (#1906) #5735

Job	Run time
wasm-unknown	2m 46s
doctest	3m 7s
docs	1m 1s
clippy	1m 28s
feature-checks	8m 33s
check no_std	1m 1s
wasm-wasi	3m 56s
test ubuntu-latest stable --no-default-features	2m 41s
fmt	13s
test ubuntu-latest stable	2m 34s
test ubuntu-latest stable --all-features	3m 7s
test ubuntu-latest nightly --no-default-features	1m 11s
test ubuntu-latest nightly	1m 33s
test ubuntu-latest nightly --all-features	1m 59s
test ubuntu-latest 1.81 --no-default-features	1m 37s
test ubuntu-latest 1.81	1m 37s
test windows-latest stable --no-default-features	5m 42s
test windows-latest stable	6m 17s
test windows-latest stable --all-features	5m 52s
test windows-latest nightly --no-default-features	3m 16s
test windows-latest nightly	3m 31s
test windows-latest nightly --all-features	4m 31s
test windows-latest 1.81 --no-default-features	3m 58s
test windows-latest 1.81	3m 43s
deny / cargo deny check	23s
ci success	3s
	1h 15m 40s

Provide feedback