Claimable Tokens Program: Add SetAuthority and Close instructions

[rickyrombo]

Adds the SetAuthority and Close instructions to the claimable tokens program.

SetAuthority is a proxy to spl_token's own set_authority. It allows the claimable tokens account to be reassigned elsewhere by setting the owner. Also allows changing the closing authority to help safeguard rent (provided the owner hasn't changed).

Note that instead of keeping a nonce account to prevent replays of these instructions, I'm using the recent blockhashes. For a bit, the instruction will be replayable. It's probably overkill actually in retrospect as setting an authority means that unsetting it now needs a signature from the existing authority.

Close is a proxy to spl_token's own close_account. It allows the rent to be reclaimed to a hardcoded address if the closeAuthority is not set and the owner is still the program PDA authority.

Risks:

• Adding a CloseAccount proxy without any guards opens us up to the DoS-style attack warned about the other proposal, I believe
• Launching both of these at the same time would allow users to manually reason the owner or closing authority before we had a chance to reclaim rent ourselves. We'll need to make sure a change goes out on Solana relay at the same time to prevent paying for opening these accounts unless we also set the close authority.

Tested using the CLI on a local solana-test-validator:

• Created a claimable token account
• Closed it
• Recreated it
• Set close authority to my wallet address
• Closing it via the program now fails
• Set owner to my wallet address
• Verified it shows up as an aux token on my wallet

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Latest commit: d8faad1

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