Optimize memory usage in SlicedMapImpl (used in BindingContext)

In short:
before these changes: Map<SlicedMapKey<slice, key>, value>
after: Map<key, Map<slice, value>> // where for nested Map used lightweight storage UserDataHolder

Before these changes it stored map from SlicedMapKey to value.
Where SlicedMapKey created for each record and store slice and key.
And now it sored map from key to UserDataHolder.
UserDataHolder store map from slice to value.

compiler/backend/src/org/jetbrains/kotlin/codegen/JvmSerializationBindings.java

@@ -17,6 +17,7 @@
 package org.jetbrains.kotlin.codegen;
 
 import com.intellij.openapi.util.Pair;
+import kotlin.Function3;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 import org.jetbrains.kotlin.descriptors.CallableMemberDescriptor;
@@ -28,7 +29,6 @@ import org.jetbrains.org.objectweb.asm.Type;
 import org.jetbrains.org.objectweb.asm.commons.Method;
 
 import java.util.Collection;
-import java.util.Map;
 
 public final class JvmSerializationBindings {
     public static final SerializationMappingSlice<FunctionDescriptor, Method> METHOD_FOR_FUNCTION =
@@ -82,13 +82,18 @@ public final class JvmSerializationBindings {
 
     @NotNull
     public static JvmSerializationBindings union(@NotNull Collection<JvmSerializationBindings> bindings) {
-        MutableSlicedMap result = SlicedMapImpl.create();
+        final MutableSlicedMap result = SlicedMapImpl.create();
+
         for (JvmSerializationBindings binding : bindings) {
-            for (Map.Entry<SlicedMapKey<?, ?>, ?> entry : binding.map) {
-                SlicedMapKey<?, ?> key = entry.getKey();
-                result.put((WritableSlice) key.getSlice(), key.getKey(), entry.getValue());
-            }
+            binding.map.forEach(new Function3<WritableSlice, Object, Object, Void>() {
+                @Override
+                public Void invoke(WritableSlice slice, Object key, Object value) {
+                    result.put(slice, key, value);
+                    return null;
+                }
+            });
         }
+
         return new JvmSerializationBindings(result);
     }