upper() and lower() should have an optional Locale argument

[rvjansen]

As described in https://sam-cooper.medium.com/the-country-that-broke-kotlin-84bdd0afb237 some locales have unexpected case folding rules which make comparisons fail. We should allow programmers to make this safe using a known locale.

[jlfaucher]

As described in https://sam-cooper.medium.com/the-country-that-broke-kotlin-84bdd0afb237

Nice story, thanks for sharing!