fix: Adds multis to map iterators, fixes searching empty nested containers (#1581)

### Summary
Eliminates `IPooledEnumerable<BaseMulti>` and `eable.Free()` from `Map` for multis. This drastically simplifies code that iterates in range, for example:

```cs
foreach (var m in m.GetMultisInRange(5))
{
}
```
The code above no longer requires an eable and calling `Free()`.


### Bug Fixes
- [X] Fixes an issue with searching through nested empty containers.

Projects/UOContent/Multis/Houses/BaseHouse.cs

@@ -1401,11 +1401,9 @@ namespace Server.Multis
                 return null;
             }
 
-            var sector = map.GetSector(loc);
-
-            for (var i = 0; i < sector.Multis.Count; ++i)
+            foreach (var house in map.GetMultisAt<BaseHouse>(loc))
             {
-                if (sector.Multis[i] is BaseHouse house && house.IsInside(loc, height))
+                if (house.IsInside(loc, height))
                 {
                     return house;
                 }