hybrid-array: add `Flatten` and `Unflatten` traits #349

Job	Run time
test (1.85.0)	43s
doc	21s
rustfmt	8s
miri (x86_64-unknown-linux-gnu)	44s
careful	1m 2s
miri (s390x-unknown-linux-gnu)	58s
clippy	19s
build (stable, armv7a-none-eabi)	48s
build (stable, wasm32-unknown-unknown)	42s
test (stable)	35s
build (1.85.0, wasm32-unknown-unknown)	52s
build (1.85.0, thumbv7em-none-eabi)	49s
build (stable, thumbv7em-none-eabi)	47s
build (1.85.0, armv7a-none-eabi)	51s
	9m 39s

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