String argument conversion + `AsArg` trait #2937

Job	Run time
doc-lints	50s
clippy	1m 52s
godot-itest (macos)	1m 7s
license-guard	35s
run-examples	1m 36s
godot-itest (windows)	2m 4s
rustfmt	16s
godot-itest (linux-full)	2m 32s
unit-test	2m 21s
godot-itest (linux-features-experimental)	2m 3s
godot-itest (linux-4.1)	1m 12s
godot-itest (linux-4.2)	1m 49s
godot-itest (linux-memcheck)	2m 23s
ci-status	1s
	20m 41s

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