[mlir][Sol] Support packed and unpacked string storage layouts #53

Job	Run time
target-machine	4s
prepare-test-matrix	59s
check-formatting (fb8689cc1fbb15616a65686b6688fcac74f921d1)	7m 15s
regression-tests (fb8689cc1fbb15616a65686b6688fcac74f921d1, default)	16m 44s
	25m 2s

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