Add more warnings to crate root #2098

Job	Run time
Check MSRV	2m 13s
Lint macOS 64bit	6m 28s
Lint iOS 32bit	1m 11s
Lint GNUStep + exceptions	2m 11s
Lint header-translator	59s
Lint test-assembly	55s
Check formatting	10s
Test macOS 12	7m 38s
Verify header translator output	1m 57s
Assembly tests	47s
${{ matrix.name }}	0s
Check icrate features	0s
Test Compiler-RT	45s
Test GNUStep with libobjc2 v1.9	2m 41s
Test GNUStep with libobjc2 v2.0	2m 42s
Test GNUStep with libobjc2 v2.1 on nightly	2m 22s
Test GNUStep 32bit	2m 35s
Compiler UI	1m 20s
Test on iOS simulator w. dinghy	0s
	36m 54s

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