libcore: char::to_digit inconsistent with char::is_digit

`to_digit` converts an ascii char of the range 0-z (or 0-Z) to a number in the range 0-35, `is_digit` on the other hand returns true if the character lies in one of the 3 numeric unicode ranges (of wich only one represents the numbers 0-9 as such).  

Possible solution: rename and create `to_digit_ascii` and `is_digit_ascii`, and move/rename `is_digit` to the `unicode`namespace?
