Iterate over corpses in active items instead of map tiles for resurrect check

[SurFlurer]

Summary

none

Purpose of change

Improve performance when there are necromancers in the reality bubble.

Describe the solution

Use get_active_items_in_radius() to grab nearby corpses only ( neglect other items, and do not iterate over tiles ). This has the advantage of less sees() and is_empty() check, which are both somewhat expensive. This also makes this function not linearly slower when there are large quantities of unrelated items nearby. ( Thanks andrei8l !)

I also made a cache for related map tile attributes to reduce calls of sees() and is_empty() if there are many corpses in one tile.

Describe alternatives you've considered

Testing

In the refugee center, with one necromancer in the backyard zombie horde, waiting for 30 minutes:

1. With this change:
   

2. Without this change:
   

Additional context

[andrei8l]

You can probably do even better by only looking at corpses, which are cached separately. NPC AI code does that:

Cataclysm-DDA/src/npcmove.cpp

Lines 3229 to 3230 in b69ae48

	for( const item_location &location : here.get_active_items_in_radius( around, range,
	special_item_type::corpse ) ) {

[SurFlurer]

You can probably do even better by only looking at corpses, which are cached separately. NPC AI code does that:

Cataclysm-DDA/src/npcmove.cpp

Lines 3229 to 3230 in b69ae48

	for( const item_location &location : here.get_active_items_in_radius( around, range,
	special_item_type::corpse ) ) {

Great thanks! I was thinking about using the active items cache, but didn't make it to find example usage of it. This is exactly what I need.

The test results did show a great improvement :)