cast6

aes: consolidate backend configuration under `aes_backend` (#541) #286

Job	Run time
test (stable)	20s
test (1.85.0)	24s
build (stable, wasm32-unknown-unknown)	17s
build (1.85.0, wasm32-unknown-unknown)	19s
build (stable, thumbv7em-none-eabi)	13s
build (1.85.0, thumbv7em-none-eabi)	21s
minimal-versions / minimal-versions	33s
	2m 27s

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