short code cleanup

[dpaiton]

Resolved Issues

Working towards #29

Description

This pulls a bunch of cleanup changes out of #116 to be reviewed independently.

• fix a bug introduced in Dry up test preamble; code cleanup #142 where test tolerances were high because the variable rate was causing short amounts to change between the rust & sol calls.
• improve docstrings & error messaging throughout
• modify solvency_after_short_derivative to return Result<T> instead of Result<Option<T>>
• modify max behavior to throw errors if there is no valid max, instead of returning 0
• add safety bounds on max short guesses to ensure it is always >= the min txn amount
• modify sol differential max short tests to use calculate_absolute_max_short instead of calculate_max_short since the solidity implementation does not consider budget
• removed the fuzz_calculate_absolute_max_short_execute in favor of a test that does the same but additionally checks that the pool is drained when that absolute max short is executed. It also includes some differential checks for rust vs solidity. This one is now called fuzz_calculate_max_short_without_budget_then_open_short
• adds a new SLOW_FUZZ_RUNS constant for one of the max tests bc it was slow as hell.

[sentilesdal]

shouldnt need to set variable to zero when advancng time.

[sentilesdal]

Just add a comment that setting the variable rate to zero is necessary to get the test to pass and that this is very bad and needs to be fixed