Add 'serde' feature for serializing and deserializing ArrayVec and ArrayString.

This implements serde support under the optional 'serde' feature, and adds unit tests to test said support.
https://serde.rs/unit-testing.html used as a guide for the unit tests - using 'serde_test' makes for much
less boilerplate here, but it does require that the project have a non-optional dev dependency on 'serde_test'.

tests/serde.rs

@@ -0,0 +1,62 @@
+#![cfg(feature = "serde")]
+extern crate arrayvec;
+extern crate serde_test;
+
+mod array_vec {
+    use arrayvec::ArrayVec;
+
+    use serde_test::{Token, assert_tokens};
+
+    #[test]
+    fn test_ser_de_empty() {
+        let vec = ArrayVec::<[u32; 0]>::new();
+
+        assert_tokens(&vec, &[
+            Token::Seq { len: Some(0) },
+            Token::SeqEnd,
+        ]);
+    }
+
+
+    #[test]
+    fn test_ser_de() {
+        let mut vec = ArrayVec::<[u32; 3]>::new();
+        vec.push(20);
+        vec.push(55);
+        vec.push(123);
+
+        assert_tokens(&vec, &[
+            Token::Seq { len: Some(3) },
+            Token::U32(20),
+            Token::U32(55),
+            Token::U32(123),
+            Token::SeqEnd,
+        ]);
+    }
+}
+
+mod array_string {
+    use arrayvec::ArrayString;
+
+    use serde_test::{Token, assert_tokens};
+
+    #[test]
+    fn test_ser_de_empty() {
+        let string = ArrayString::<[u8; 0]>::new();
+
+        assert_tokens(&string, &[
+            Token::Str(""),
+        ]);
+    }
+
+
+    #[test]
+    fn test_ser_de() {
+        let string = ArrayString::<[u8; 9]>::from("1234 abcd")
+            .expect("expected exact specified capacity to be enough");
+
+        assert_tokens(&string, &[
+            Token::Str("1234 abcd"),
+        ]);
+    }
+}