Move `async-signature` and `signature-derive` to toplevel (#1391) #2136

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	3m 31s
build (1.65.0, wasm32-unknown-unknown)	5m 11s
test (1.65.0)	6m 17s
build (stable, thumbv7em-none-eabi)	23s
test (stable)	28s
build (stable, wasm32-unknown-unknown)	21s
	16m 11s

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