add option to use DST structs for flex arrays #1992

Job	Run time
msrv	5m 13s
minimal	12s
check-cfg	24s
docs	29s
quickchecking	16s
rustfmt-clippy	31s
test-book	13s
test (ubuntu-latest, 9.0, 1, 0, 0, 0, 0)	2m 16s
test-expectations (ubuntu-latest)	16s
test (ubuntu-latest, 9.0, 1, 0, 0, 1, 0)	2m 17s
test-expectations (macos-latest)	32s
test (ubuntu-latest, 9.0, 1, 0, 1, 0, 0)	1m 54s
test (ubuntu-latest, 9.0, 1, 0, 1, 1, 0)	2m 15s
test (ubuntu-latest, 9.0, 1, 1, 0, 0, 0)	2m 14s
test (ubuntu-latest, 9.0, 1, 1, 0, 1, 0)	2m 18s
test (ubuntu-latest, 9.0, 1, 1, 1, 0, 0)	2m 35s
test (ubuntu-latest, 9.0, 1, 1, 1, 1, 0)	2m 41s
test (ubuntu-latest, 16.0, 1, 0, 0, 0, 0)	3m 0s
test (ubuntu-latest, 16.0, 1, 0, 0, 1, 0)	2m 44s
test (ubuntu-latest, 16.0, 1, 0, 1, 0, 0)	2m 39s
test (ubuntu-latest, 16.0, 1, 0, 1, 1, 0)	2m 39s
test (ubuntu-latest, 16.0, 1, 1, 0, 0, 0)	3m 27s
test (ubuntu-latest, 16.0, 1, 1, 0, 1, 0)	3m 27s
test (ubuntu-latest, 16.0, 1, 1, 1, 0, 0)	3m 17s
test (ubuntu-latest, 16.0, 1, 1, 1, 1, 0)	3m 6s
test (ubuntu-latest, 16.0, 0, 0, 1)	2m 27s
test (macos-latest, 9.0, 0, 0, 0)	2m 29s
success	1s
	55m 52s

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