- Remove the `Registry`, which only allows one `Pool` per asset pair + tick + lot size, and module initializer. This simplifies the code quite a bit
- Add `PoolCreated` event
- Make all events record the object ID of the relevant pool
- Put the pool creation fee inside an inaccessible field inside the `Pool` type instead of burning
- Move the trading fees to an inaccessible field inside the `Pool` type
- Introduce separate functions for rounding and non-rounding math
- Move usage of `AccountCap` closer to sensitive operations
- Introduce `custodian::lock_balance` and `custodian::unlock_balance` to ensure atomic movement of funds to/from unlocked and locked states
- Make all relevant `custodian` and `critbit` functions `friend` to give us max upgrade flexibility
- Rename `usr_balance` for consistency with other `balance` functions (which start with `account)`
- Use `object::id` whenever possible