This makes it so that if you're operating on chars or wchars with
std.ascii functions, you don't have to cast the return values back to
char or wchar, but it still allows you to operate on dchars without
mangling any Unicode values.
Given that these functions probably aren't used very often, we'd
probably be better of making their names longer and more descriptive as
Lars suggested. So, they're now isPrintible, isGraphical, and
isPunctuation.
isWhite, isLower, isUpper, toLower, and toUpper now have Ascii in their
name, which matches what std.unit does with its versions of those
functions. Hopefully, it should also reduce bugs due to using the wrong
function between the ASCII and unicode versions by making the difference
more obvious.
