Redo instance variables #2352

Job	Run time
Check formatting	8s
Check MSRV	1m 23s
Lint macOS 64bit	2m 38s
Lint iOS 32bit	54s
Lint GNUStep + exceptions	1m 21s
Lint header-translator	34s
Lint test-assembly	27s
Test macOS 12	5m 0s
Verify header translator output	1m 3s
Compiler UI	4m 36s
Assembly tests	48s
Check icrate features	0s
Test GNUStep with libobjc2 v1.9	1m 27s
Test Compiler-RT	19s
Test GNUStep with libobjc2 v2.0	1m 30s
Test GNUStep with libobjc2 v2.1 on nightly	1m 57s
Test on iOS simulator w. dinghy	0s
Test GNUStep 32bit	1m 26s
${{ matrix.name }}	0s
	25m 31s

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