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/isa/riscv/recipes.rs

@@ -25,7 +25,8 @@ impl RecipeGroup {
     fn push(&mut self, builder: EncodingRecipeBuilder) {
         assert!(
             self.name_to_recipe.get(&builder.name).is_none(),
-            format!("riscv recipe '{}' created twice", builder.name)
+            "riscv recipe '{}' created twice",
+            builder.name
         );
         let name = builder.name.clone();
         let number = self.recipes.push(builder.build());