Implement conversion between signature types #3627

Job	Run time
cfg-check	1m 14s
clippy	53s
docs	1m 53s
doctest	1m 23s
fmt	12s
feature-checks	3m 30s
no-std	18s
test ubuntu-latest stable --no-default-features	1m 40s
wasm-unknown	1m 22s
wasm-wasi	1m 32s
test ubuntu-latest stable	1m 33s
test ubuntu-latest stable --all-features	2m 18s
test ubuntu-latest nightly --no-default-features	2m 35s
test ubuntu-latest nightly	2m 28s
test ubuntu-latest nightly --all-features	2m 50s
test ubuntu-latest 1.76 --no-default-features	44s
test ubuntu-latest 1.76	43s
test windows-latest stable --no-default-features	3m 27s
test windows-latest stable	4m 20s
test windows-latest stable --all-features	4m 49s
test windows-latest nightly --no-default-features	5m 57s
test windows-latest nightly	6m 4s
test windows-latest nightly --all-features	6m 20s
test windows-latest 1.76 --no-default-features	2m 16s
test windows-latest 1.76	2m 15s
	1h 2m 36s

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