Add metadata field to prefab attributes / fix performance regression for classes without annotations #144

Job	Run time
test-prefab (ubuntu-latest, 3.10)	10s
test-stubs	18s
test-prefab (ubuntu-latest, 3.11)	17s
test-prefab (ubuntu-latest, 3.12)	14s
test-prefab (ubuntu-latest, 3.8)	19s
test-prefab (ubuntu-latest, 3.9)	10s
test-prefab (ubuntu-latest, 3.13)	9s
test-prefab (ubuntu-latest, pypy-3.10)	33s
	2m 10s

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