crypto_box: derive `Copy` for `PublicKey` (#173) #283

Job	Run time
build (1.60.0, thumbv7em-none-eabi)	52s
test (1.60.0)	1m 0s
test (stable)	15s
build (1.60.0, wasm32-unknown-unknown)	54s
build (stable, thumbv7em-none-eabi)	7s
build (stable, wasm32-unknown-unknown)	8s
	3m 16s

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