Store bank_seed on Bank struct #414
Description
Problem
We use bank_seed to derive Bank PDAs, but don't store it on the Bank struct. This means we can't re-derive the PDAs later when looking at it onchain.
It also means we're never able to re-validate the bank struct's PDA or any other PDAs that are derived using the bank's seed in later instructions.
Solution
Add pub bank_seed: u64 field to Bank struct and store it in all add_pool instructions.