Add WarningDiagnostic::emit (#668) #917

Job	Run time
docs	1m 52s
clippy	2m 1s
rustfmt	13s
runefmt	1m 49s
basics	1s
msrv	1m 54s
miri_rune_alloc	4m 7s
build_feature (capture-io)	1m 1s
no_default_features	54s
build_feature (cli,doc)	1m 32s
miri_rune	5m 33s
no_std_minimal	3m 5s
build_feature (cli,fmt)	1m 41s
build_feature (cli)	1m 32s
build_feature (workspace)	1m 1s
wasm	58s
build_feature (languageserver)	1m 13s
build_feature (byte-code)	1m 1s
test	10m 47s
	42m 15s

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