Add downcasting capability based on `isKindOfClass:` #2100

Job	Run time
Check formatting	12s
Check MSRV	1m 35s
Lint macOS 64bit	7m 47s
Lint iOS 32bit	1m 36s
Lint GNUStep + exceptions	2m 24s
Lint header-translator	44s
Lint test-assembly	32s
Test macOS 12	49s
Verify header translator output	2m 9s
Assembly tests	1m 44s
${{ matrix.name }}	0s
Check icrate features	0s
Test Compiler-RT	48s
Compiler UI	2m 13s
Test on iOS simulator w. dinghy	0s
Test GNUStep with libobjc2 v1.9	2m 24s
Test GNUStep with libobjc2 v2.0	4m 33s
Test GNUStep with libobjc2 v2.1 on nightly	3m 36s
Test GNUStep 32bit	3m 28s
	36m 34s

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