Expose `Field<Type>::get_attrs,get_type`#36

I can't find any reason to make the `get_attrs` method only available on `Field<Value>` but not `Field<Type>`. It's very useful when we try to derive a method without self receiver, e.g.: ```rust mod schema { type Field; type VecField; trait Fields { fn fields() -> VecField; } } ``` It is very likely that it was simply forgotten by #22. @eupn Can you give some suggestions? Thanks!