Compute monster toHit on the fly

[StephenCWills]

For context, refer to #7433 (reply in thread) and the comments that follow.

Because monster order has changed between vanilla and the current version of DevilutionX, we can't rely on the fact that monster toHit was already initialized. Otherwise, we will end up using the wrong monster's toHit value if we load a vanilla savegame.

It seems the reason we don't use the value from the save file is because we don't want to inherit erroneous values from vanilla due to the bug that applies difficulty bonuses twice. In that case, it's better to compute the value on the fly, like with Monster::level().

However, we can't entirely do away with the toHit variable because a Golem's hit chance depends on the player character's stats at the time the spell was cast. I had to keep the variable but renamed it to golemToHit to avoid naming conflicts. The toHit() function simply returns the golemToHit value if the monster is a golem.