add option to use DST structs for flex arrays #1971

Job	Run time
msrv	4m 8s
minimal	11s
rustfmt-clippy	13s
docs	27s
check-cfg	39s
quickchecking	19s
test-book	6s
test (ubuntu-latest, 9.0, 1, 0, 0, 0, 0)	2m 20s
test-expectations (ubuntu-latest)	1m 2s
test (ubuntu-latest, 9.0, 1, 0, 0, 1, 0)	2m 25s
test-expectations (macos-latest)	1m 47s
test (ubuntu-latest, 9.0, 1, 0, 1, 0, 0)	3m 37s
test (ubuntu-latest, 9.0, 1, 0, 1, 1, 0)	2m 24s
test (ubuntu-latest, 9.0, 1, 1, 0, 0, 0)	2m 10s
test (ubuntu-latest, 9.0, 1, 1, 0, 1, 0)	3m 56s
test (ubuntu-latest, 9.0, 1, 1, 1, 0, 0)	2m 15s
test (ubuntu-latest, 9.0, 1, 1, 1, 1, 0)	2m 21s
test (ubuntu-latest, 16.0, 1, 0, 0, 0, 0)	3m 44s
test (ubuntu-latest, 16.0, 1, 0, 0, 1, 0)	4m 14s
test (ubuntu-latest, 16.0, 1, 0, 1, 0, 0)	2m 58s
test (ubuntu-latest, 16.0, 1, 0, 1, 1, 0)	3m 6s
test (ubuntu-latest, 16.0, 1, 1, 0, 0, 0)	3m 40s
test (ubuntu-latest, 16.0, 1, 1, 0, 1, 0)	5m 29s
test (ubuntu-latest, 16.0, 1, 1, 1, 0, 0)	3m 1s
test (ubuntu-latest, 16.0, 1, 1, 1, 1, 0)	2m 57s
test (ubuntu-latest, 16.0, 0, 0, 1)	1m 47s
test (macos-latest, 9.0, 0, 0, 0)	1m 22s
success	1s
	1h 2m 39s

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