better managed ownership (two new functions in level.h)

src/level.cc

@@ -237,14 +237,35 @@ position level::get_down_stairs() const {
         return map.get_down_stairs();
 }
 
-enemy_list &level::get_elist() {
+const enemy_list &level::get_elist() {
         return elist;
 }
 
-potion_list &level::get_plist() {
+const potion_list &level::get_plist() {
         return plist;
 }
 
 gold_list &level::get_glist() {
         return glist;
 }
+
+std::unique_ptr<potion> level::detach_potion(size_t idx) {
+        if (idx >= plist.size())
+                return nullptr;
+
+        auto tmp = std::move(pplist[idx]);
+        plist.erase(plist.begin() + idx);
+        pplist.erase(pplist.begin() + idx);
+        return tmp;
+}
+
+void level::erase_enemy(character *ch) {
+        if (ch == nullptr)
+                return;
+
+        for (size_t i = 0; i < elist.size(); ++i)
+                if (elist[i] == ch) {
+                        elist.erase(elist.begin() + i);
+                        pelist.erase(pelist.begin() + i);
+                }
+}