Bring back Airdrop

[ChihChengLiang]

We used to have Airdrop in the Reddit competition, but we removed it from the codebase to keep the scope simple.

Design space

• 4 bytes sender index
• 2 bytes compressed value that each receiver will receive

There are choices we can specify the receivers

• if receivers are existing users with states: specify each stateID
• if receivers have pubkey registered but have no state leaves: specify pubkeyIDs and a state subtree node.

TODO

Think about the overhead of the fraudproof