refactor: remove `Signature` generic (#719) #1956

Job	Run time
miri --no-default-features	4m 16s
miri	4m 15s
test ubuntu-latest stable --no-default-features	59s
miri --all-features	5m 33s
test ubuntu-latest stable	1m 2s
test ubuntu-latest beta --no-default-features	57s
clippy	55s
test ubuntu-latest beta	58s
docs	55s
test ubuntu-latest nightly --no-default-features	1m 14s
feature-checks	5m 14s
test ubuntu-latest nightly	1m 18s
fmt	11s
test ubuntu-latest 1.79 --no-default-features	27s
wasm	36s
test ubuntu-latest 1.79	29s
test windows-latest stable --no-default-features	2m 10s
test windows-latest stable	2m 23s
test windows-latest beta --no-default-features	1m 51s
test windows-latest beta	1m 50s
test windows-latest nightly --no-default-features	2m 39s
test windows-latest nightly	2m 43s
test windows-latest 1.79 --no-default-features	1m 4s
test windows-latest 1.79	1m 20s
test ubuntu-latest 1.79 --features json	37s
test windows-latest 1.79 --features json	1m 9s
test ubuntu-latest stable --all-features	1m 24s
test ubuntu-latest beta --all-features	1m 27s
test ubuntu-latest nightly --all-features	1m 23s
	51m 19s

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