Renamed AsciiStr::to_lower and AsciiStr::to_upper
Now AsciiStr::to_lowercase and AsciiStr::to_uppercase to match Ascii trait.
[breaking-change]
auto merge of #17947 : lukemetz/rust/master, r=aturon

AsciiStr::to_lower is now AsciiStr::to_lowercase and AsciiStr::to_upper is AsciiStr::to_uppercase to match Ascii trait.

Part of issue #17790.

This is my first pull request so let me know if anything is incorrect.

Thanks!

[breaking-changes]