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.

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