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'.

.travis.yml

@@ -1,5 +1,7 @@
 language: rust
 sudo: false
+env:
+  - FEATURES='serde'
 matrix:
   include:
     - rust: 1.12.0
@@ -15,7 +17,7 @@ matrix:
       - NODROP_FEATURES='use_needs_drop'
     - rust: nightly
       env:
-      - FEATURES='use_union'
+      - FEATURES='serde use_union'
       - NODROP_FEATURES='use_union'
 branches:
   only: