make Fixnum::build_with harder to accidentally misuse

change trait bound order for better

src/machine/loader.rs

@@ -2344,7 +2344,9 @@ impl Machine {
                             MetaSpec::Either => atom_as_cell!(atom!("?")),
                             MetaSpec::Colon => atom_as_cell!(atom!(":")),
                             MetaSpec::RequiresExpansionWithArgument(ref arg_num) => {
-                                fixnum_as_cell!(Fixnum::build_with(*arg_num as i64))
+                                fixnum_as_cell!(/* FIXME this is not safe */ unsafe {
+                                    Fixnum::build_with_unchecked(*arg_num as i64)
+                                })
                             }
                         });
                     }