add option to use DST structs for flex arrays #1985

Job	Run time
minimal	12s
docs	20s
check-cfg	21s
msrv	4m 15s
quickchecking	16s
rustfmt-clippy	29s
test (ubuntu-latest, 9.0, 1, 0, 0, 0, 0)	1m 59s
test-book	19s
test (ubuntu-latest, 9.0, 1, 0, 0, 1, 0)	1m 43s
test-expectations (ubuntu-latest)	42s
test (ubuntu-latest, 9.0, 1, 0, 1, 0, 0)	1m 48s
test-expectations (macos-latest)	1m 1s
test (ubuntu-latest, 9.0, 1, 0, 1, 1, 0)	1m 53s
test (ubuntu-latest, 9.0, 1, 1, 0, 0, 0)	2m 1s
test (ubuntu-latest, 9.0, 1, 1, 0, 1, 0)	2m 0s
test (ubuntu-latest, 9.0, 1, 1, 1, 0, 0)	2m 3s
test (ubuntu-latest, 9.0, 1, 1, 1, 1, 0)	2m 1s
test (ubuntu-latest, 16.0, 1, 0, 0, 0, 0)	2m 3s
test (ubuntu-latest, 16.0, 1, 0, 0, 1, 0)	2m 3s
test (ubuntu-latest, 16.0, 1, 0, 1, 0, 0)	2m 3s
test (ubuntu-latest, 16.0, 1, 0, 1, 1, 0)	2m 3s
test (ubuntu-latest, 16.0, 1, 1, 0, 0, 0)	2m 2s
test (ubuntu-latest, 16.0, 1, 1, 0, 1, 0)	2m 2s
test (ubuntu-latest, 16.0, 1, 1, 1, 0, 0)	2m 2s
test (ubuntu-latest, 16.0, 1, 1, 1, 1, 0)	2m 2s
test (ubuntu-latest, 16.0, 0, 0, 1)	1m 51s
test (macos-latest, 9.0, 0, 0, 0)	1m 30s
success	1s
	43m 5s

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