Change compilation unit to enum #4914

Job	Run time
build test linux x86-64	5m 53s
check-rust	4m 5s
build test windows x86-64	9m 37s
check-website	10s
scarb-metadata compatibility check	59s
test linux x86-64 1/8	3m 32s
test linux x86-64 2/8	4m 9s
test linux x86-64 3/8	4m 3s
test linux x86-64 4/8	3m 6s
test linux x86-64 5/8	3m 38s
test linux x86-64 6/8	2m 18s
test linux x86-64 7/8	2m 5s
test linux x86-64 8/8	1m 45s
test windows x86-64 1/8	3m 54s
test windows x86-64 2/8	3m 58s
test windows x86-64 3/8	3m 58s
test windows x86-64 4/8	3m 2s
test windows x86-64 5/8	3m 52s
test windows x86-64 6/8	2m 19s
test windows x86-64 7/8	2m 19s
test windows x86-64 8/8	1m 51s
	1h 10m 33s

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