crypto_box: derive `Copy` for `PublicKey` #348

Job	Run time
test (1.60.0)	57s
build (1.60.0, thumbv7em-none-eabi)	40s
test (stable)	24s
build (1.60.0, wasm32-unknown-unknown)	42s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 10s
cross (aarch64-unknown-linux-gnu, stable)	1m 10s
build (stable, thumbv7em-none-eabi)	15s
cross (powerpc-unknown-linux-gnu, stable)	1m 14s
build (stable, wasm32-unknown-unknown)	16s
	6m 48s

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