Rename functions in Ascii

Some functions implemented for the Ascii struct have the same functionality as other functions implemented for the normal chars. For consistency, I think they should have the same name, so I renamed the functions in Ascii to match the names in the Char trait.

* Renamed `to_lower` to `to_lowercase`
* Renamed `to_upper` to `to_uppercase`
* Renamed `is_alpha` to `is_alphabetic`
* Renamed `is_alnum` to `is_alphanumeric`
* Renamed `is_lower` to `is_lowercase`
* Renamed `is_upper` to `is_uppercase`

[breaking-change]