argon2: improve `const` compatibility (#438) #757

Job	Run time
build (1.60.0, thumbv7em-none-eabi)	1m 38s
build (1.60.0, wasm32-unknown-unknown)	1m 33s
build (stable, thumbv7em-none-eabi)	32s
build (stable, wasm32-unknown-unknown)	25s
test (1.60.0)	1m 43s
test (stable)	33s
minimal-versions / minimal-versions	43s
	7m 7s

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