Added gcc option '-Wconversion'

No warnings for standard numerical types. Still pending alternative
numerical types.

llex.c

@@ -350,7 +350,7 @@ static unsigned long readutf8esc (LexState *ls) {
   int i = 4;  /* chars to be removed: '\', 'u', '{', and first digit */
   save_and_next(ls);  /* skip 'u' */
   esccheck(ls, ls->current == '{', "missing '{'");
-  r = gethexa(ls);  /* must have at least one digit */
+  r = cast_ulong(gethexa(ls));  /* must have at least one digit */
   while (cast_void(save_and_next(ls)), lisxdigit(ls->current)) {
     i++;
     esccheck(ls, r <= (0x7FFFFFFFu >> 4), "UTF-8 value too large");