Custom blob subtypes fixes #679

Job	Run time
run-guard	1s
linting	42s
embedded (linking)	1m 15s
embedded (dynamic_loading)	1m 38s
testing (ubuntu-20.04, v3, linking)	4m 8s
testing (ubuntu-20.04, v3, dynamic_loading)	4m 21s
security	11s
testing (ubuntu-20.04, v3, pure_rust)	5m 43s
testing (ubuntu-20.04, v2, linking)	5m 40s
testing (ubuntu-20.04, v2, dynamic_loading)	5m 13s
testing (ubuntu-20.04, v2, pure_rust)	5m 38s
testing (ubuntu-20.04, v4, linking)	4m 31s
testing (ubuntu-20.04, v4, dynamic_loading)	4m 3s
testing (ubuntu-20.04, v4, pure_rust)	6m 7s
testing (windows-latest, v4, linking)	4m 2s
testing (windows-latest, v4, dynamic_loading)	3m 50s
testing (windows-latest, v4, pure_rust)	4m 7s
testing (macos-latest, v3, linking)	51s
testing (macos-latest, v3, dynamic_loading)	1m 42s
testing (macos-latest, v3, pure_rust)	1m 57s
coverage	3s
	1h 5m 43s

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