Use `impl` instead of ProtocolObject where possible #3139

Job	Run time
Lint macOS x86_64	29s
Lint iOS ARMv7s	41s
Check MSRV	18s
Check formatting	13s
Lint visionOS Aarch64 simulator	41s
Lint GNUStep + exceptions	26s
Lint header-translator	22s
Lint test-assembly	25s
Test macOS 15 Aarch64	33s
Assembly tests	0s
Check framework features	0s
Verify header translator output	0s
${{ ((matrix.test && 'Test') \|\| 'Build') }} ${{ matrix.name }}	0s
${{ matrix.name }}	0s
Test on iOS simulator w. dinghy	0s
Compiler UI	0s
Test Compiler-RT	0s
	4m 8s

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