Make pulping corpses and using pickaxe into player activities.

Includes cleanup of player_activity, moving that class into separate files, cleanup of processing activities in game.cpp

fixes #5262
fixes #5858

I have a documentation on how to create new activities (where should I put this?):

Adding new activities:

1. Add an entry to `activity_type`, right before `NUM_ACTIVITIES`.
2. Add a stop phrase in `player_activity::get_stop_phrase` for the new activity. Use the existing phrases as example.
3. Write an `on_turn_<activity>` function and/or an `on_finish_<activity>` function and add a case statement in `game::activity_on_turn` and/or `game::activity_on_finish` to call that function(when it's processing the new activity).
4. Start the activity by calling `u.assign_activity(ACT_<activity>, ...)`

Notes:
An activity is considered finished when `player_activity::moves_left` is at or below 0. One can reach this state either by explicit setting the value to 0, or over time by decreasing the value each turn.

Activities can be canceled by calling `game::cancel_activity` or similar. Canceling an activity prevents the `game::activity_on_finish` function from running, the activity does therefor not yield a result. If the activity is finished normally (`player_activity::moves_left <= 0`), the `game::activity_on_finish` is called.
Canceling the activity witch `cancel_activity` allows for resuming the activity as a copy of the activity is written to `player::backlog` (only if `player_activity::is_suspendable()` returns true).

An activity can be stopped by setting `player_activity::type` to `ACT_NULL`, but this does not allow resuming the task. It is usually used to signal that the activity has been finished early.

While the PC performs the activity, `game::activity_on_turn` is called on each turn. This functions calls some activity specific functions and decrements `player_activity::moves_left`, so it will eventually reach 0.

Some activities (like playing on a PDA) don't have an end result but instead have do something each turn (playing on the PDA decrease its batteries and gives moral bonus).

If the activity needs any information during its execution or when it's finished (like where to do something, which item to use to do something, ...), use any of
- `player_activity::index` - what to do (which recipe to craft, ...)
- `player_activity::position` - item position in players inventory (player::i_at)
- `player_activity::name` - name of something to do (skill to be trained)
- `player_activity::values`
- `player_activity::str_values`
- `player_activity::placement` - where to do something
Those values are automatically saved and restored when loading a game. Other than that they are not changed/examined by any common code. Different types of activities can use them differently - for example index or placement.x can be used as item position by on activity and as something different by other activities.

Notes on new `on_turn_<activity>` functions:
Make sure that 
- `player_activity::moves_left` is decreased (either in `on_turn_<activity>` or directly in `game::activity_on_turn`),
- or that the activity is stopped there.

For example the pulp activity finishes itself when there are no more unpulped corpses on the map. It therefor does not need to decrease `player_activity::moves_left` on each turn.

Notes on new `on_finish_<activity>` functions:
This function is called when the activity has been completed, it must set the `player_activity::type` to ACT_NULL (or assign a new activity). One should not call `cancel_activity` for the finished activity, as this could make a copy of the activity in `player::backlog`, which allows resuming an already finished activity.

One might also add a case for the new activity in `player_activity::is_suspendable`, `player_activity::is_abortable` (see documentation of those functions for what they do).