[Swift] Rebuild the way swift handles structs from scratch (#6326)

* Rebuild the way swift handles structs from scratch

* Updates docs, and sample binary

* Replaces InMemory to Mutable

* Migrates docs from inmemory

* use inline for some functions

* Renamed Mutable objects

* Updates documentation

swift/Sources/FlatBuffers/UnionObject.swift

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2020 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import Foundation
+
+public protocol UnionObject {}
+
+extension UnionObject {
+
+  /// Serialize is a helper function that serailizes the data from the Object API to a bytebuffer directly th
+  /// - Parameter type: Type of the Flatbuffer object
+  /// - Returns: returns the encoded sized ByteBuffer
+  public func serialize<T: ObjectAPI>(type: T.Type) -> ByteBuffer where T.T == Self {
+    var builder = FlatBufferBuilder(initialSize: 1024)
+    return serialize(builder: &builder, type: type.self)
+  }
+
+  /// Serialize is a helper function that serailizes the data from the Object API to a bytebuffer directly.
+  ///
+  /// - Parameters:
+  ///   - builder: A FlatBufferBuilder
+  ///   - type: Type of the Flatbuffer object
+  /// - Returns: returns the encoded sized ByteBuffer
+  /// - Note: The `serialize(builder:type)` can be considered as a function that allows you to create smaller builder instead of the default `1024`.
+  ///  It can be considered less expensive in terms of memory allocation
+  public func serialize<T: ObjectAPI>(builder: inout FlatBufferBuilder, type: T.Type) -> ByteBuffer where T.T == Self {
+    var s = self
+    let root = type.pack(&builder, obj: &s)
+    builder.finish(offset: root)
+    return builder.sizedBuffer
+  }
+}