Implement Serialize/Deserialize for ID scalar

As per the unofficial Rust API guidelines:

https://rust-lang-nursery.github.io/api-guidelines/interoperability.html#data-structures-implement-serdes-serialize-deserialize-c-serde

juniper/CHANGELOG.md

@@ -1,6 +1,6 @@
 # master
 
-- No changes yet
+- The `ID` scalar now implements Serde's `Serialize` and `Deserialize`
 
 # [[0.12.0] 2019-05-16](https://github.com/graphql-rust/juniper/releases/tag/juniper-0.12.0)
 

juniper/src/types/scalars.rs

@@ -1,3 +1,4 @@
+use serde_derive::{Deserialize, Serialize};
 use std::convert::From;
 use std::marker::PhantomData;
 use std::ops::Deref;
@@ -13,7 +14,7 @@ use crate::value::{ParseScalarResult, ScalarRefValue, ScalarValue, Value};
 /// An ID as defined by the GraphQL specification
 ///
 /// Represented as a string, but can be converted _to_ from an integer as well.
-#[derive(Clone, Debug, Eq, PartialEq)]
+#[derive(Clone, Debug, Eq, PartialEq, Serialize, Deserialize)]
 pub struct ID(String);
 
 impl From<String> for ID {