add option to use DST structs for flex arrays #1989

Job	Run time
msrv	4m 37s
docs	18s
rustfmt-clippy	27s
check-cfg	23s
minimal	17s
quickchecking	14s
test-book	12s
test (ubuntu-latest, 9.0, 1, 0, 0, 0, 0)	2m 13s
test-expectations (ubuntu-latest)	37s
test-expectations (macos-latest)	48s
test (ubuntu-latest, 9.0, 1, 0, 0, 1, 0)	4m 13s
test (ubuntu-latest, 9.0, 1, 0, 1, 0, 0)	2m 27s
test (ubuntu-latest, 9.0, 1, 0, 1, 1, 0)	3m 47s
test (ubuntu-latest, 9.0, 1, 1, 0, 0, 0)	2m 44s
test (ubuntu-latest, 9.0, 1, 1, 0, 1, 0)	2m 13s
test (ubuntu-latest, 9.0, 1, 1, 1, 0, 0)	2m 15s
test (ubuntu-latest, 9.0, 1, 1, 1, 1, 0)	2m 27s
test (ubuntu-latest, 16.0, 1, 0, 0, 0, 0)	3m 26s
test (ubuntu-latest, 16.0, 1, 0, 0, 1, 0)	2m 37s
test (ubuntu-latest, 16.0, 1, 0, 1, 0, 0)	2m 58s
test (ubuntu-latest, 16.0, 1, 0, 1, 1, 0)	3m 41s
test (ubuntu-latest, 16.0, 1, 1, 0, 0, 0)	3m 35s
test (ubuntu-latest, 16.0, 1, 1, 0, 1, 0)	3m 11s
test (ubuntu-latest, 16.0, 1, 1, 1, 0, 0)	3m 31s
test (ubuntu-latest, 16.0, 1, 1, 1, 1, 0)	3m 21s
test (ubuntu-latest, 16.0, 0, 0, 1)	2m 18s
test (macos-latest, 9.0, 0, 0, 0)	1m 33s
success	1s
	1h 0m 24s

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