Use IItemHandler instead of IInventory internally (#2971)

* Use IItemHandler instead of IInventory internally

src/main/java/appeng/util/Lazy.java

@@ -0,0 +1,44 @@
+/*
+ * This file is part of Applied Energistics 2.
+ * Copyright (c) 2013 - 2017, AlgorithmX2, All rights reserved.
+ *
+ * Applied Energistics 2 is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Applied Energistics 2 is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with Applied Energistics 2.  If not, see <http://www.gnu.org/licenses/lgpl>.
+ */
+
+package appeng.util;
+
+
+import java.util.function.Supplier;
+
+
+public class Lazy<T> implements Supplier<T>
+{
+	private final Supplier<T> supplier;
+	private T instance = null;
+
+	public Lazy( final Supplier<T> supplier )
+	{
+		this.supplier = supplier;
+	}
+
+	@Override
+	public T get()
+	{
+		if( instance == null )
+		{
+			instance = supplier.get();
+		}
+		return instance;
+	}
+}