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'.

Cargo.toml

@@ -19,6 +19,14 @@ version = "0.1.8"
 path = "nodrop"
 default-features = false
 
+[dependencies.serde]
+version = "1.0"
+optional = true
+default-features = false
+
+[dev-dependencies.serde_test]
+version = "1.0"
+
 [features]
 default = ["std"]
 std = ["odds/std", "nodrop/std"]