Resolve needless_borrow clippy lint

    error: this expression borrows a reference (`&syn::FieldsNamed`) that is immediately dereferenced by the compiler
      --> derive/src/ser.rs:13:37
       |
    13 |         }) => derive_struct(&input, &fields),
       |                                     ^^^^^^^ help: change this to: `fields`
       |
       = note: `-D clippy::needless-borrow` implied by `-D clippy::all`
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

derive/src/ser.rs

@@ -10,7 +10,7 @@ pub fn derive(input: DeriveInput) -> Result<TokenStream> {
         Data::Struct(DataStruct {
             fields: Fields::Named(fields),
             ..
-        }) => derive_struct(&input, &fields),
+        }) => derive_struct(&input, fields),
         Data::Enum(enumeration) => derive_enum(&input, enumeration),
         _ => Err(Error::new(
             Span::call_site(),