Help doing opt-in of ASA to smart contract

[fabrice]

See Why have both txn.sender() and txn.asset_sender() in PyTEAL? - #2 by jannotti

In short:

• the receiver for an Algo payment transaction is Receiver
• the receiver for an ASA transfer is AssetReceiver
• the sender is always Sender, except for ASA clawback/revoke transactions (Algorand Standard Assets (ASAs) - Algorand Developer Portal)