ignore unused_parens warning

parens are generated by external macro

src/machine/copier.rs

@@ -1,3 +1,5 @@
+#![allow(unused_parens)] // see mthom/scryer-prolog#3092 and rust-lang/rust#147126
+
 use fxhash::FxBuildHasher;
 use indexmap::IndexSet;