Symbols may now include digits in lexer

This is mostly so labels get to have digits.  This won't affect number
tokens as that happens before symbols.

asm/lexer.c

@@ -136,7 +136,8 @@ char uppercase(char c)
 
 bool is_symbol(char c)
 {
-  return isalpha(c) || c == '-' || c == '_' || c == '.' || c == ':';
+  return isalpha(c) || isdigit(c) || c == '-' || c == '_' || c == '.' ||
+         c == ':';
 }
 
 bool is_valid_hex_char(char c)