feat: use `OnceCell` for `Signed::hash` #6019

Job	Run time
doctest	1m 27s
wasm-unknown	35s
check no_std	12s
clippy	24s
docs	57s
feature-checks	25s
feature-propagation	50s
test ubuntu-latest stable --no-default-features	1m 35s
wasm-wasi	41s
test ubuntu-latest stable	2m 9s
fmt	14s
test ubuntu-latest stable --all-features	2m 38s
test ubuntu-latest nightly --no-default-features	1m 32s
test ubuntu-latest nightly	1m 32s
test ubuntu-latest nightly --all-features	2m 31s
test ubuntu-latest 1.81 --no-default-features	46s
test ubuntu-latest 1.81	1m 1s
test windows-latest stable --no-default-features	4m 49s
test windows-latest stable	6m 36s
test windows-latest stable --all-features	6m 17s
test windows-latest nightly --no-default-features	4m 32s
test windows-latest nightly	4m 1s
test windows-latest nightly --all-features	5m 36s
test windows-latest 1.81 --no-default-features	2m 28s
test windows-latest 1.81	2m 44s
deny / cargo deny check	19s
ci success	3s
	56m 54s

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