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

[TennyZhuang]

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.:

            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!