pkcs5: simple `pbes2::Params` constructors #330

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 51s
build (1.65.0, wasm32-unknown-unknown)	1m 51s
build (stable, thumbv7em-none-eabi)	31s
build (stable, wasm32-unknown-unknown)	25s
test (1.65.0)	1m 58s
test (stable)	29s
minimal-versions / minimal-versions	40s
	7m 45s

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