Update isdigit, isalpha, and isalnum to handle Unicode without debug assertions

[sjanzou]

Issue with debug assertions when using isdigit, isalpha lk functions processing various coordinate formats:

Running this script in Debug mode in Windows, I get an assertion failed error from the LK isdigit() function in lk\src\stdlib.cpp. The script runs fine in Release mode.

x = '51°30′26″N 0°7′39″W';
outln(x);

for (i=0; i<strlen(x); i++ ) {
c = ch(x,i);
d = isdigit(c);
a = isalpha(c);
outln(c + ' ' + d + ' ' + a);
}