Add `flatten`, `flatten_with_order` and `into_flat` to arrays #207

# to view logs

Job	Run time
docs/${{ matrix.rust }}	0s
cargo-careful	0s
cross_test/${{ matrix.target }}/${{ matrix.rust }}	0s
clippy/stable	23s
format/nightly	11s
nostd-build/thumbv6m-none-eabi/stable	9s
tests/stable	2m 40s
tests/beta	2m 46s
tests/nightly	3m 8s
tests/1.64.0	5m 47s
conclusion	1s
	15m 5s

Provide feedback