hybrid-array: add `Flatten` and `Unflatten` traits (#170) #350

Job	Run time
doc	14s
clippy	21s
careful	1m 0s
rustfmt	9s
test (1.85.0)	41s
test (stable)	35s
miri (s390x-unknown-linux-gnu)	59s
build (stable, armv7a-none-eabi)	46s
build (1.85.0, wasm32-unknown-unknown)	51s
build (1.85.0, thumbv7em-none-eabi)	48s
build (stable, wasm32-unknown-unknown)	58s
miri (x86_64-unknown-linux-gnu)	42s
build (stable, thumbv7em-none-eabi)	47s
build (1.85.0, armv7a-none-eabi)	51s
	9m 42s

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