WIP: Codegen option to create module hierarchy #1949

Job	Run time
linux nightly (all features)	13m 0s
Check sync-readme	1m 27s
linux stable (default features)	10m 34s
linux beta (default features)	10m 28s
linux stable (with-bytes)	10m 11s
mega-linter	3m 45s
Miri test	55s
rustfmt check	8m 35s
CI self-check	8s
windows stable (default features)	4m 14s
	1h 3m 17s

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