Implement support for oneofs (C unions).

Basic test included, should probably add an oneof to the AllTypes test also.

Update issue 131
Status: Started

tests/oneof/oneof.proto

@@ -0,0 +1,18 @@
+import 'nanopb.proto';
+
+message SubMessage
+{
+    repeated int32 array = 1 [(nanopb).max_count = 8];
+}
+
+message OneOfMessage
+{
+    required int32 prefix = 1;
+    oneof values
+    {
+        int32 first = 5;
+        string second = 6 [(nanopb).max_size = 8];
+        SubMessage third = 7;
+    }
+    required int32 suffix = 99;
+}