Fix a number of warnings on nightly Rust (#2652)

This fixes some issues that are cropping up where some syntax will get
phased out in 2021

cranelift/codegen/meta/src/cdsl/encodings.rs

@@ -153,10 +153,9 @@ impl EncodingBuilder {
         let inst = self.inst.inst();
         assert!(
             Rc::ptr_eq(&inst.format, &recipes[self.recipe].format),
-            format!(
-                "Inst {} and recipe {} must have the same format!",
-                inst.name, recipes[self.recipe].name
-            )
+            "Inst {} and recipe {} must have the same format!",
+            inst.name,
+            recipes[self.recipe].name
         );
 
         assert_eq!(