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/Server/Items/Container.Enumerable.cs

@@ -161,14 +161,17 @@ public partial class Container
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private bool SetNextContainer()
         {
-            if (!_containers.TryDequeue(out var c))
+            while (_containers.TryDequeue(out var c))
             {
-                return false;
+                _items = CollectionsMarshal.AsSpan(c.m_Items);
+                _index = 0;
+                if (SetNextItem())
+                {
+                    return true;
+                }
             }
 
-            _items = CollectionsMarshal.AsSpan(c.m_Items);
-            _index = 0;
-            return SetNextItem();
+            return false;
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]