Add downcasting capability based on `isKindOfClass:` #2070

Job	Run time
Check formatting	9s
Check MSRV	1m 42s
Lint macOS 64bit	7m 10s
Lint iOS 32bit	1m 11s
Lint GNUStep + exceptions	1m 58s
Lint header-translator	35s
Lint test-assembly	37s
Test macOS 12	5m 39s
Verify header translator output	3m 35s
Assembly tests	52s
Test GNUStep with libobjc2 v1.9	2m 46s
Test GNUStep with libobjc2 v2.0	2m 17s
Test GNUStep with libobjc2 v2.1 on nightly	3m 16s
Test GNUStep 32bit	2m 27s
Check icrate features	0s
Compiler UI	1m 17s
Test Compiler-RT	31s
Test on iOS simulator w. dinghy	0s
${{ matrix.name }}	0s
	36m 2s

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