H98: unicode

[Ross Paterson]

Another fiddly Unicode point:

   uniDigit   -> any Unicode numeric

This allows things like roman numerals, superscripts, subscripts and
fractions in numerical literals.  I suggest

   uniDigit   -> any Unicode decimal digit

One could mix digits from different languages, but the meaning would
be clear (the Unicode tables include the digit value).  An alternative
would be to drop uniDigit; octal or hexadecimal numerals are already
limited to ASCII.