add option to use DST structs for flex arrays #1981

Job	Run time
docs	21s
quickchecking	26s
rustfmt-clippy	30s
msrv	4m 11s
test-expectations (ubuntu-latest)	15s
test (ubuntu-latest, 9.0, 1, 0, 0, 0, 0)	1m 35s
test-expectations (macos-latest)	39s
test (ubuntu-latest, 9.0, 1, 0, 0, 1, 0)	1m 40s
test (ubuntu-latest, 9.0, 1, 0, 1, 0, 0)	1m 39s
check-cfg	23s
test (ubuntu-latest, 9.0, 1, 0, 1, 1, 0)	1m 40s
test (ubuntu-latest, 9.0, 1, 1, 0, 0, 0)	1m 39s
minimal	18s
test (ubuntu-latest, 9.0, 1, 1, 0, 1, 0)	1m 40s
test (ubuntu-latest, 9.0, 1, 1, 1, 0, 0)	1m 46s
test (ubuntu-latest, 9.0, 1, 1, 1, 1, 0)	1m 37s
test-book	5s
test (ubuntu-latest, 16.0, 1, 0, 0, 0, 0)	1m 40s
test (ubuntu-latest, 16.0, 1, 0, 0, 1, 0)	1m 39s
test (ubuntu-latest, 16.0, 1, 0, 1, 0, 0)	1m 39s
test (ubuntu-latest, 16.0, 1, 0, 1, 1, 0)	1m 41s
test (ubuntu-latest, 16.0, 1, 1, 0, 0, 0)	1m 40s
test (ubuntu-latest, 16.0, 1, 1, 0, 1, 0)	1m 39s
test (ubuntu-latest, 16.0, 1, 1, 1, 0, 0)	1m 41s
test (ubuntu-latest, 16.0, 1, 1, 1, 1, 0)	1m 42s
test (ubuntu-latest, 16.0, 0, 0, 1)	1m 39s
test (macos-latest, 9.0, 0, 0, 0)	1m 24s
success	1s
	36m 49s

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