Rework the safety invariants of `Global` a bit #2364

Job	Run time
doc-lints	49s
license-guard	30s
godot-itest (macos)	58s
godot-itest (windows)	2m 37s
clippy	1m 31s
godot-itest (linux-full)	2m 15s
run-examples	1m 15s
rustfmt	26s
godot-itest (linux-features-experimental)	2m 7s
unit-test	2m 22s
godot-itest (linux-4.1)	1m 33s
godot-itest (linux-memcheck)	2m 39s
godot-itest (linux-memcheck-4.0)	2m 5s
ci-status	2s
	21m 9s

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