Make settings::Flags::new consume the Builder.

This makes it more clear what the relationship is between the Builder
and the resulting Flags.

lib/frontend/src/lib.rs

@@ -117,7 +117,7 @@
 //!         builder.finalize();
 //!     }
 //!
-//!     let flags = settings::Flags::new(&settings::builder());
+//!     let flags = settings::Flags::new(settings::builder());
 //!     let res = verify_function(&func, &flags);
 //!     println!("{}", func.display(None));
 //!     match res {