reclaim buffer account

[kaze-cow]

Description

Adds the ReclaimBuffers instruction. Also add some more capabilities to the test helpers.

Stacked on #84, which adds the receiver field to the state PDA that this instruction reads. Review that one first; the diff here is against it.

Changes

The instruction takes in 3 accounts, state_account, receiver_account, token_account, plus two additional accounts buffer_pda, mint for each buffer to clear. The mint is required to help verify the canonical status of buffer_pda, as well as to call Burn if the account happens to hold excess funds.

The instruction can only be called by the receiver account which is registered in the state_pda (see #84).

Since a bad actor could send 1 wei/lamport of a token to the buffers just before we close them, the buffer may become unable to be closed and the instruction would fail. This could be used as a griefing tactic, so this instruction will additionally Burn any remaining tokens found in the buffer before closing. Burn is used instead of Transfer because a transfer would likely require conditional allocation and sending--all for what is almost certainly a tiny . So its much easier to simply burn this amount entirely and move on. As this is a potentially destructive operation, multiple warnings have been added around the codebase to prevent this from being overlooked. Additionally, while most tokens support anyone being able to Burn their own tokens, there may exist certain 2022 tokens which disallow it, and so the buffer could not be closed unless we were to empty the account completely.

How to test

• Confirm ReclaimBuffer general methodology.
• Consider behavior of burning tokens as a way of reducing the security impact

[linear-code]

SC-151

[fedgiac]

The PR is fairly large so I'd suggest to split it in 2: first, introducing the new state PDA data and the initialization; then, introducing the new reclaim buffer instruction. If that's large as well you can consider splitting parsing/ix building to the processing but maybe it isn't needed (the burn logic is nontrivial though).