Clean up aim_activity_actor

[olanti-p]

Summary

SUMMARY: None

Purpose of change

Simplifies aim_activity_actor as a follow-up after #40684
Fixes #42250

The crash happened due to the following coincidence:

• aiming was finished with player having exactly 0 moves left
• the woodbow had 1 turn left before expiring

aim_activity_actor::do_turn(...) method requires an extra call for marking the activity as finished, so in a case when aiming was finished with player having exactly 0 moves left, that extra call was delayed by 1 turn, and so aim_activity_actor::finish(...) was also delayed by 1 turn.

That 1-turn delay was enough for the woodbow to disappear.

But, finish(...) method assumed that the weapon checks done back in do_turn(...) were still relevant, so it simply dereferenced the weapon pointer, which have turned into a null pointer by that time, without validation.

Describe the solution

Removed the need for an extra call by inserting act.moves_left = 0 in do_turn(...) wherever necessary.
Added a nullptr check in finish(...) for robustness in the future.
Also, noticed that my old implementation used shared pointers instead of (de-)serializable and more convenient cata::optional, so changed that.
Removed an enum that was basically a leftover from refactoring.

Describe alternatives you've considered

Just slapping a nullptr check, but that's treating symptoms, not the cause.

Testing

Set (via debugger) avatar's moves and the woodbow's timer to expire as described. The game crashed before, but after this fix the bow allows the last shot before disappearing.