Improve optional support for borsh serialization

- Do not allocate when deserializing ArrayString
- Serialize length as u32, not as u64, to be consistent with
  serialization of [T] and str
- Add tests

.github/workflows/ci.yml

@@ -18,7 +18,7 @@ jobs:
       matrix:
         include:
           - rust: 1.51.0 # MSRV
-            features: serde
+            features: serde, borsh
             experimental: false
           - rust: stable
             features:
@@ -28,7 +28,7 @@ jobs:
             features: serde
             experimental: false
           - rust: nightly
-            features: serde, zeroize
+            features: serde, borsh, zeroize
             experimental: false
 
     steps: