Fixing Char.is_lower and Char.is_upper

[o-santi]

The characters between 65 and 91 are the uppercase letters. The characters between 97 and 127 are the lowercase letters.

The way it was, Char.is_lower accepts any codepoint that is not an uppercase letter, and conversely Char.is_upper accepts any codepoint that is not a lowercase letter.

[algebraic-dev]

You have commited a file called SelfTypes/_.kind2, I guess that this file is out of the scope of the PR.