add option to use DST structs for flex arrays #1970

Job	Run time
minimal	10s
check-cfg	22s
docs	31s
msrv	4m 11s
rustfmt-clippy	21s
test-book	11s
test (ubuntu-latest, 9.0, 1, 0, 0, 0, 0)	2m 25s
test-expectations (ubuntu-latest)	1m 2s
test (ubuntu-latest, 9.0, 1, 0, 0, 1, 0)	1m 45s
quickchecking	14s
test-expectations (macos-latest)	1m 36s
test (ubuntu-latest, 9.0, 1, 0, 1, 0, 0)	2m 13s
test (ubuntu-latest, 9.0, 1, 0, 1, 1, 0)	2m 12s
test (ubuntu-latest, 9.0, 1, 1, 0, 0, 0)	2m 55s
test (ubuntu-latest, 9.0, 1, 1, 0, 1, 0)	3m 26s
test (ubuntu-latest, 9.0, 1, 1, 1, 0, 0)	2m 51s
test (ubuntu-latest, 9.0, 1, 1, 1, 1, 0)	2m 43s
test (ubuntu-latest, 16.0, 1, 0, 0, 0, 0)	4m 16s
test (ubuntu-latest, 16.0, 1, 0, 0, 1, 0)	2m 36s
test (ubuntu-latest, 16.0, 1, 0, 1, 0, 0)	2m 42s
test (ubuntu-latest, 16.0, 1, 0, 1, 1, 0)	2m 59s
test (ubuntu-latest, 16.0, 1, 1, 0, 0, 0)	3m 34s
test (ubuntu-latest, 16.0, 1, 1, 0, 1, 0)	3m 27s
test (ubuntu-latest, 16.0, 1, 1, 1, 0, 0)	3m 35s
test (ubuntu-latest, 16.0, 1, 1, 1, 1, 0)	3m 1s
test (ubuntu-latest, 16.0, 0, 0, 1)	2m 20s
test (macos-latest, 9.0, 0, 0, 0)	1m 20s
success	1s
	58m 59s

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