Wrap max long & targeted long to something like calculate_max_valid_long(max_budget, target_rate)

[dpaiton]

We want an easy-to-use fn that always returns a valid long that is maximally profitable. For example:

if fixed_rate > variable_rate {
  target_rate = variable_rate;
  agent.open_long(hyperdrive.max_valid_long(agent.budget, target_rate);
}

The max_valid_long fn would be guaranteed to be valid, and return a long that is as close to the target as possible without exceeding it. In the event that the long to achieve the target rate produces an insolvent pool, it will return the maximum long.

This is different from the existing calculate_targeted_long, which will throw an error if the long to hit the target_rate produces an insolvent pool.

[dpaiton]

Blocked by #21