Refactor OptAppAndData

[matthiasgeihs]

In actual, nothing is optional. Both App and Data must be non-nil values. What happens internally is:

1. If the app is noApp: for encoding, we encode false and just return. for decoding, we read the false value and set app, data to noApp and noData repectively.
2. Else: for encoding, we encode true, followed by the app and data. for decoding: we read the true value and then decode the app and data.

With the above understanding, I think a single struct called AppAndData with Encode and Decode methods defined on it will be a better approach.

However, this was an existing construct, which I only moved it from one package to another.
Because, this PR in itself is somewhat large, if we need to modify this, I think we should do in a separate one.

Originally posted by @manoranjith in #272 (comment)