Add, and use, macros to do locale-independent case mapping.

Add, and use, macros to do locale-independent case mapping.

This means we get the same behavior in Turkish locales (where, if we
aren't in the C locale, we might get lower-case "i" mapped to upper-case
"I with dot" and upper-case "I" mapped to lower-case "i without dot),
and may also suppress some shortening warnings from MSVC.