Add `flatten`, `flatten_with_order` and `into_flat` to arrays #141

Job	Run time
docs/${{ matrix.rust }}	0s
cargo-careful	0s
clippy/beta	18s
cross_test/${{ matrix.target }}/${{ matrix.rust }}	0s
format/nightly	14s
tests/stable	3m 53s
tests/beta	4m 6s
tests/nightly	4m 31s
tests/1.57.0	7m 32s
conclusion	1s
	20m 35s

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