deny_public_fields: display field identifier in help message

Use a binding's field identifier to produce more descriptive output.
Fall back to a binding's identifier if a field identifier is not
available.

Additionally, make the privatize.rs compiletest more specific and
wrap the identifier in backticks to aid readability.

components/deny_public_fields/lib.rs

@@ -17,7 +17,8 @@ fn expand_string(input: &str) -> String {
     let style = synstructure::BindStyle::Ref.into();
     synstructure::each_field(&type_, &style, |binding| {
         if binding.field.vis != syn::Visibility::Inherited {
-            panic!("Field {} should not be public", binding.ident);
+            panic!("Field `{}` should not be public",
+                   binding.field.ident.as_ref().unwrap_or(&binding.ident));
         }
         "".to_owned()
     });

tests/compiletest/plugin/compile-fail/privatize.rs

@@ -9,6 +9,7 @@ extern crate deny_public_fields;
 
 #[derive(DenyPublicFields)]
 //~^ ERROR proc-macro derive panicked
+//~| HELP Field `v1` should not be public
 struct Foo {
     pub v1: i32,
     v2: i32