Track kills through grenades and other explosives

The source of the explosion for grenades was being passed as the
character they were (potentially) on, because no other character was
available to be responsible at that point. To make a character
available, whenever an item is activated or thrown, set an item var with
the last character to do so on it. Then, use that to find the explosion
source.

Also, ignore it in checking if an item stacks with another.

src/item.cpp

@@ -1714,7 +1714,7 @@ stacking_info item::stacks_with( const item &rhs, bool check_components, bool co
     bits.set( tname::segments::UPS, _stacks_ups( *this, rhs ) );
     // Guns that differ only by dirt/shot_counter can still stack,
     // but other item_vars such as label/note will prevent stacking
-    static const std::set<std::string> ignore_keys = { "dirt", "shot_counter", "spawn_location_omt", "ethereal" };
+    static const std::set<std::string> ignore_keys = { "dirt", "shot_counter", "spawn_location_omt", "ethereal", "last_act_by_char_id" };
     bits.set( tname::segments::VARS, map_equal_ignoring_keys( item_vars, rhs.item_vars, ignore_keys ) );
     bits.set( tname::segments::ETHEREAL, _stacks_ethereal( *this, rhs ) );
     bits.set( tname::segments::LOCATION_HINT, _stacks_location_hint( *this, rhs ) );

src/iuse_actor.cpp

@@ -241,6 +241,8 @@ std::optional<int> iuse_transform::use( Character *p, item &it, const tripoint &
         return std::nullopt;
     }
 
+    it.set_var( "last_act_by_char_id", p->getID().get_value() );
+
     int result = 0;
 
     if( need_fire ) {
@@ -652,10 +654,19 @@ void explosion_iuse::load( const JsonObject &obj, const std::string & )
     obj.read( "scrambler_blast_radius", scrambler_blast_radius );
 }
 
-std::optional<int> explosion_iuse::use( Character *p, item &, const tripoint &pos ) const
+std::optional<int> explosion_iuse::use( Character *p, item &it, const tripoint &pos ) const
 {
     if( explosion.power >= 0.0f ) {
-        explosion_handler::explosion( p, pos, explosion );
+        Character *source = p;
+        if( it.has_var( "last_act_by_char_id" ) ) {
+            character_id thrower( it.get_var( "last_act_by_char_id", 0 ) );
+            if( thrower == get_player_character().getID() ) {
+                source = &get_player_character();
+            } else {
+                source = g->find_npc( thrower );
+            }
+        }
+        explosion_handler::explosion( source, pos, explosion );
     }
 
     if( draw_explosion_radius >= 0 ) {

src/ranged.cpp

@@ -1416,6 +1416,7 @@ dealt_projectile_attack Character::throw_item( const tripoint_bub_ms &target, co
 {
     // Copy the item, we may alter it before throwing
     item thrown = to_throw;
+    thrown.set_var( "last_act_by_char_id", getID().get_value() );
 
     const int move_cost = throw_cost( *this, to_throw );
     mod_moves( -move_cost );