add option to use DST structs for flex arrays #1983

Job	Run time
docs	27s
check-cfg	28s
minimal	20s
msrv	4m 15s
quickchecking	14s
rustfmt-clippy	32s
test-book	10s
test (ubuntu-latest, 9.0, 1, 0, 0, 0, 0)	3m 46s
test-expectations (ubuntu-latest)	42s
test (ubuntu-latest, 9.0, 1, 0, 0, 1, 0)	1m 52s
test-expectations (macos-latest)	1m 2s
test (ubuntu-latest, 9.0, 1, 0, 1, 0, 0)	1m 52s
test (ubuntu-latest, 9.0, 1, 0, 1, 1, 0)	2m 0s
test (ubuntu-latest, 9.0, 1, 1, 0, 0, 0)	2m 36s
test (ubuntu-latest, 9.0, 1, 1, 0, 1, 0)	2m 23s
test (ubuntu-latest, 9.0, 1, 1, 1, 0, 0)	2m 37s
test (ubuntu-latest, 9.0, 1, 1, 1, 1, 0)	2m 38s
test (ubuntu-latest, 16.0, 1, 0, 0, 0, 0)	2m 38s
test (ubuntu-latest, 16.0, 1, 0, 0, 1, 0)	3m 10s
test (ubuntu-latest, 16.0, 1, 0, 1, 0, 0)	2m 59s
test (ubuntu-latest, 16.0, 1, 0, 1, 1, 0)	2m 34s
test (ubuntu-latest, 16.0, 1, 1, 0, 0, 0)	3m 55s
test (ubuntu-latest, 16.0, 1, 1, 0, 1, 0)	3m 23s
test (ubuntu-latest, 16.0, 1, 1, 1, 0, 0)	3m 8s
test (ubuntu-latest, 16.0, 1, 1, 1, 1, 0)	3m 42s
test (ubuntu-latest, 16.0, 0, 0, 1)	1m 48s
test (macos-latest, 9.0, 0, 0, 0)	2m 15s
success	1s
	57m 27s

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