Input does not support UTF8

[fusetim]

Short description:
Using any UTF8 character that is not an ASCII char in an input will panic!

Why does it happen?
We use numerous method on String that can split or get a char that is not a UTF8 char boundary, therefore creating an invalid substring.

What should we do?
Use graphems and not chars.
Indeed even if we fix the original issue by checking or finding the closest UTF8 char boundary, it might introduce some weird behavior when using extended graphems (like emoji modifiers).