audit: reduce cognitive overhead

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auction.sol is complicated. Contributors need to remember a fair bit of constraints. One possible change is to recombine the assertions and the business logic to make it clearer what the limits are on variables as they are used.

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Defer for the short term as the problem with this is in having new contributors write secure code, which is harder with the current cognitive load.

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I'm not sure if it makes it any simpler, but one way of doing this is to combine all of the contracts into a single large one. I've done this as part of trying to debug dapple 0.8 and it comes to ~800 lines.