Make Signature::new a const fn #2171

Job	Run time
check no_std	31s
clippy	45s
docs	31s
features stable	1m 34s
fmt	12s
features stable --all-targets	2m 59s
miri --no-default-features	4m 56s
features nightly	1m 51s
miri	4m 55s
features nightly --all-targets	3m 2s
wasm	25s
miri --all-features	7m 21s
test ubuntu-latest stable --no-default-features	1m 4s
test ubuntu-latest stable	1m 0s
test ubuntu-latest nightly --no-default-features	57s
test ubuntu-latest nightly	54s
test ubuntu-latest 1.79 --no-default-features	30s
test ubuntu-latest 1.79	30s
test windows-latest stable --no-default-features	4m 5s
test windows-latest stable	2m 38s
test windows-latest nightly --no-default-features	2m 14s
test windows-latest nightly	2m 11s
test windows-latest 1.79 --no-default-features	1m 22s
test windows-latest 1.79	1m 2s
test ubuntu-latest 1.79 --features json	30s
test windows-latest 1.79 --features json	1m 6s
test ubuntu-latest nightly --all-features	1m 13s
test windows-latest nightly --all-features	2m 26s
ci success	4s
	52m 48s

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