Release notes for 4.0

• #66 Adds the final keyword to all classes preventing subclassing
• #70 Introduces native types where they were previously missing. Most of these changes will not cause any impact now that the classes are final.
• #71 removes the deprecated ImplicitMethodsIntegrationTest - this abstract test case was renamed for PHPUnit compatibility to AbstractImplicitMethodsIntegrationTest. This change will only affect integration tests for Router implementations
• #68 changes the constructor signature of ImplicitHeadMiddleware - it now only accepts a PSR-17 StreamFactoryInterface where previously, it also accepted an arbitrary callable.
• #69 changes the constructor signatures of MethodNotAllowedMiddleware and ImplicitOptionsMiddleware. Previously, both of these middleware accepted a callable and a PSR-17 ResponseFactoryInterface. The ability to pass a callable has been removed
• Some internal, and now unused traits and classes have been removed. As these were not part of the public API, their removal should not cause any problems.
• #72 Changes the way that configuration is processed for the the detection of duplicate routes.
  Previously an array was expected in the format:

  return [
    \Mezzio\Router\RouteCollector::class => [
      'detect_duplicates' => true,
    ],
  ];

  Now, the top level key of router should be used:

  return [
    'router' => [
      'detect_duplicates' => true,
    ],
  ];

  Duplicate route detection still defaults to true so this change will affect users that explicitly turned off duplicate route detection, and, where duplicate routes exist, or the feature was turned off explictly to save the additional overhead in production.