Attempt to fix #3 (at least partially).

debug_backtrace supports only one level of backtrace
but here we store the line of the last instruction
then go through the byte code.

include/ph7int.h

@@ -1105,6 +1105,7 @@ struct VmInstr {
 	sxu8  iOp; /* Operation to preform */
 	sxi32 iP1; /* First operand */
 	sxu32 iP2; /* Second operand (Often the jump destination) */
+	sxu32 iLine; /* Line number */
 	void *p3;  /* Third operand (Often Upper layer private data) */
 };
 /* Each active class instance attribute is represented by an instance