[Swift] Inline arrays (#8755)

Implements InlineArrays which allow us to use Flatbuffers arrays within
Structs natively, and also implements FlatbufferVectors as a secondary API
when using mutable Structs

Fixes mutations within fixed sizes arrays

Adds tests and remove inout and mutating from generated objects in favor of borrowing

---------

Co-authored-by: Wouter van Oortmerssen <aardappel@gmail.com>

swift/Sources/FlatBuffers/String+extension.swift

@@ -88,7 +88,7 @@ extension String: ObjectAPIPacker {
     builder.create(string: obj)
   }
 
-  public mutating func unpack() -> String {
+  public func unpack() -> String {
     self
   }