Basic implementation of WTON contract.

Interaction with different kinds of assets requires additional conditional logic. This contract unifies interaction with assets and splits assets from paying for gas.

The contract is compatible to TEP-74 and TEP-89 standards.

Implementation is based on locking TON coins on minter contract on minting jettons and releasing TON on burning jettons.

The contract implement WTON specific operations:

burn_notification_ext#84106950 query_id:uint64 amount:Coins
                           sender:MsgAddress response_destination:MsgAddress
                           custom_payload:(Maybe ^Cell) = InternalMsgBody;

mint#864e0716 query_id:uint64 amount:Coins recipient:MsgAddress forward_amount:Coins
              forward_payload:(Maybe ^Cell) return_excesses_to:MsgAddress = InternalMsgBody;

release#71c6af6b query_id:uint64 custom_payload:(Maybe ^Cell) = InternalMsgBody;

It reserves amount + minimal_balance() TON on the minter contract and sends the rest to a WTON wallet of recipient within internal_transfer message. One of the most important things is an ability to attach forward_amount and forward_payload to build a pipeline of transactions.

The message should be rejected if:

• msg_value is less than amount + forward_amount + gas_fee
• recipient is in different workchain than the minter

In order to unwrap tokens from jettons to TON, burn operation should be used. Current relies on existing operation burn (including custom_payload) but uses a new operation burn_notification_ext instead of burn_notification. The only reason of replacing burn_notification by a new burn_notification_ext is inability to attach a custom_payload.

After burning, a jetton wallet sends burn_notification_ext to a minter. As soon as a minter receive burn_notification_ext message, it decreased total_supply by amount, reserves total_supply + minimal_balance() and sends a message release to response_destination attaching the rest of TON and optional custom_payload.

The message burn should be rejected if:

• response_destination is not specified
• amount is less than jetton balance of jetton wallet
• msg_sender is not an owner of jetton wallet
• msg_value is not enough for handling both messages burn and burn_notification_ext