Add readData(int) to read primitive value.

javanano/src/main/java/com/google/protobuf/nano/CodedInputByteBufferNano.java

@@ -638,4 +638,44 @@ public final class CodedInputByteBufferNano {
       throw InvalidProtocolBufferNanoException.truncatedMessage();
     }
   }
+
+  // Read a primitive type.
+  Object readData(int type) throws IOException {
+    switch (type) {
+      case InternalNano.TYPE_DOUBLE:
+          return readDouble();
+      case InternalNano.TYPE_FLOAT:
+          return readFloat();
+      case InternalNano.TYPE_INT64:
+          return readInt64();
+      case InternalNano.TYPE_UINT64:
+          return readUInt64();
+      case InternalNano.TYPE_INT32:
+          return readInt32();
+      case InternalNano.TYPE_FIXED64:
+          return readFixed64();
+      case InternalNano.TYPE_FIXED32:
+          return readFixed32();
+      case InternalNano.TYPE_BOOL:
+          return readBool();
+      case InternalNano.TYPE_STRING:
+          return readString();
+      case InternalNano.TYPE_BYTES:
+          return readBytes();
+      case InternalNano.TYPE_UINT32:
+          return readUInt32();
+      case InternalNano.TYPE_ENUM:
+          return readEnum();
+      case InternalNano.TYPE_SFIXED32:
+          return readSFixed32();
+      case InternalNano.TYPE_SFIXED64:
+          return readSFixed64();
+      case InternalNano.TYPE_SINT32:
+          return readSInt32();
+      case InternalNano.TYPE_SINT64:
+          return readSInt64();
+      default:
+          throw new IllegalArgumentException("Unknown type " + type);
+    }
+  }
 }