Add String::{to_lowercase,to_uppercase} #921

Job	Run time
clippy	1m 58s
rustfmt	14s
runefmt	1m 50s
docs	1m 35s
basics	1s
miri_rune_alloc	4m 3s
no_default_features	50s
miri_rune	5m 24s
build_feature (capture-io)	57s
msrv	2m 2s
no_std_minimal	2m 9s
build_feature (cli,doc)	1m 43s
build_feature (cli,fmt)	3m 4s
wasm	1m 8s
build_feature (cli)	1m 33s
build_feature (workspace)	53s
build_feature (languageserver)	1m 25s
build_feature (byte-code)	55s
test	11m 30s
	43m 14s

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