add option to use DST structs for flex arrays #2027

Job	Run time
check-cfg	22s
minimal	12s
docs	32s
msrv	1m 38s
quickchecking	16s
rustfmt-clippy	32s
test (ubuntu-latest, 9.0, 1, 0, 0, 0, 0)	2m 6s
test-book	20s
test (ubuntu-latest, 9.0, 1, 0, 0, 1, 0)	2m 14s
test-expectations (ubuntu-latest)	1m 5s
test (ubuntu-latest, 9.0, 1, 0, 1, 0, 0)	2m 50s
test-expectations (macos-latest)	2m 8s
test (ubuntu-latest, 9.0, 1, 0, 1, 1, 0)	2m 28s
test (ubuntu-latest, 9.0, 1, 1, 0, 0, 0)	2m 33s
test (ubuntu-latest, 9.0, 1, 1, 0, 1, 0)	2m 48s
test (ubuntu-latest, 9.0, 1, 1, 1, 0, 0)	2m 37s
test (ubuntu-latest, 9.0, 1, 1, 1, 1, 0)	2m 21s
test (ubuntu-latest, 16.0, 1, 0, 0, 0, 0)	3m 11s
test (ubuntu-latest, 16.0, 1, 0, 0, 1, 0)	3m 30s
test (ubuntu-latest, 16.0, 1, 0, 1, 0, 0)	3m 10s
test (ubuntu-latest, 16.0, 1, 0, 1, 1, 0)	4m 17s
test (ubuntu-latest, 16.0, 1, 1, 0, 0, 0)	3m 28s
test (ubuntu-latest, 16.0, 1, 1, 0, 1, 0)	3m 32s
test (ubuntu-latest, 16.0, 1, 1, 1, 0, 0)	3m 20s
test (ubuntu-latest, 16.0, 1, 1, 1, 1, 0)	3m 30s
test (ubuntu-latest, 16.0, 0, 0, 1)	2m 22s
test (macos-latest, 9.0, 0, 0, 0)	2m 12s
success	1s
	59m 35s

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