Replace accounts with keys in ProposalInstruction

[metaproph3t]

Something annoying is that TypeScript generates [TransactionInstruction](https://solana-labs.github.io/solana-web3.js/classes/TransactionInstruction.html)s, which have programId, keys, and data, while Rust instructions have programId, accounts, and data. Since we're going to be mostly building instructions from TypeScript, I think we should move to be using keys.