ENHANCED: allow Roman numerals in strings

Example:

    ?- X = "ↁ".
       X = "ↁ".

This addresses #1790.

src/parser/macros.rs

@@ -20,7 +20,7 @@ macro_rules! alpha_char {
 #[macro_export]
 macro_rules! alpha_numeric_char {
     ($c: expr) => {
-        $crate::alpha_char!($c) || $crate::decimal_digit_char!($c)
+        $crate::alpha_char!($c) || $c.is_numeric()
     };
 }