ssh-cipher: make `Encryptor` and `Decryptor` generic around cipher (#… #1196

Job	Run time
test (1.85.0)	3m 51s
test (stable)	3m 40s
no_std (stable, thumbv7em-none-eabi)	1m 24s
no_std (stable, wasm32-unknown-unknown)	1m 33s
no_std (1.85.0, thumbv7em-none-eabi)	1m 48s
no_std (1.85.0, wasm32-unknown-unknown)	1m 39s
build (x86_64-apple-darwin, macos-latest, stable)	36s
build (x86_64-pc-windows-msvc, windows-latest, stable)	1m 25s
build (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mul...	54s
	16m 50s

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