Reimplement import() builtin function as a global-scope statement. This fixes #58.

include/ph7int.h

@@ -1376,6 +1376,7 @@ enum iErrCode {
 enum ph7_vm_op {
 	PH7_OP_DONE =   1,   /* Done */
 	PH7_OP_HALT,         /* Halt */
+	PH7_OP_IMPORT,       /* Import AerScript module */
 	PH7_OP_INCLUDE,      /* Include another source file */
 	PH7_OP_DECLARE,      /* Declare a variable */
 	PH7_OP_LOADV,        /* Load variable */