Fixed Price Batch Auction

[Oighty]

After reviewing various scenarios, it has become clear that atomic auctions are not compatible with several important direct to liquidity use cases. Therefore, we need a batch auction format of the Fixed Price Sale as well.

This format is different from most batch auctions that we could write since the price is already determined at the beginning of the "auction". This means that once we receive enough bids to fill the capacity at the fixed price, we can end the auction early.

The following additional design decisions were made to improve user experience and provide flexibility in the use of the format.

• There is no max bid amount. This can be enforced by a callback and likely users want more nuanced control (e.g. limiting the amount each address can buy)
• There is a minimum fill percent, similar to EMP.
• If a user's bid payout causes the auction to reach capacity, the auction ends immediately and can be settled.
• If a user's bid payout exceeds the capacity of the auction, they receive a partial fill instead of reverting and the auctions ends as above.
• We do not need to iterate through bids to determine a settlement value, so we do not store the bid IDs in an array (requiring some view functions to not be implemented).

[Oighty]

There's a bit of duplication between FPB and EMP in this initial version (data structures, errors, revert functions, etc.). Not sure if we want to deal with refactoring EMP, but something to consider while reviewing.

[etherSky111]

All good.