'Numeric' is the proper name of the unicode character class,
and this frees up the word 'digit' for ascii use in libcore.
Since I'm going to rename `Char::is_digit_radix` to
`is_digit`, I am not leaving a deprecated method in place,
because that would just cause name clashes, as both
`Char` and `UnicodeChar` are in the prelude.
[breaking-change]