argon2: improve `const` compatibility (#438) #339

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 55s
build (1.65.0, wasm32-unknown-unknown)	2m 3s
build (stable, thumbv7em-none-eabi)	35s
build (stable, wasm32-unknown-unknown)	31s
test (1.65.0)	1m 50s
test (stable)	38s
minimal-versions / minimal-versions	1m 8s
	8m 40s

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