UnicodeString

[RealyUniqueName]

Closes #8175

• Introduces UnicodeString which consistently handles all unicode code points across all targets.
• Removes haxe.Utf8
• Adds neko.Utf8

This implementation is not locale-aware, which means that for some languages toUpperCase/toLowerCase may work incorrectly.
E.g. for turkish "ıi".toUpperCase() becomes "II instead of "Iİ".

[RealyUniqueName]

Cpp failure is unrelated.

@ncannasse hl fails on these lines (other targets pass):

s > s2;
s >= s2;
s2 < s;
s2 <= s;

wheres s is UnicodeString and s2 is String.

[RealyUniqueName]

@ncannasse hl fails on these lines (other targets pass):

That's not related to UnicodeString: HaxeFoundation/hashlink#250

[RealyUniqueName]

@hughsando The same issue with hxcpp: HaxeFoundation/hxcpp#797