Some improvements to FrugalList

[stephentoub]

Description

• FrugalStructList's ICollection<T>-based constructor uses foreach to enumerate the contents of the collection. If it's an IList<T>, we can instead index and avoid allocating the enumerator.
• Avoid multiple interface calls to ICollection<T>.Count in FrugalStructList's ctor
• Delete a dead ctor on ArrayItemList<T>. That ctor was the only reason an array field may have been left null, so we can also remove subsequent null checks when accessing that array.
• Use Span/Array in ArrayItemList for Clear, Contains, IndexOf, ToArray, and CopyTo rather than open-coding them

Customer Impact

Unnecessary allocation and interface dispatch

Regression

No

Testing

CI

Risk

Minimal

[bgrainger]

I haven't tracked down the issue yet, but when I cherry-picked this PR against release/6.0 (to produce a custom build of WPF for a .NET 6.0 application), it crashed at startup with the following exception:

System.ArgumentNullException: Value cannot be null. (Parameter 'genericHandler')
   at System.Windows.RoutedEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget) in /_/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/RoutedEventArgs.cs:line 285
   at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target) in /_/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/RoutedEventArgs.cs:line 336
   at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised) in /_/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/EventRoute.cs:line 207
   at System.Windows.EventRoute.InvokeHandlers(Object source, RoutedEventArgs args) in /_/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/EventRoute.cs:line 128
   at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args) in /_/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/UIElement.cs:line 2340
   at System.Windows.UIElement.RaiseEvent(RoutedEventArgs e) in /_/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Generated/UIElement.cs:line 419
   at System.Windows.BroadcastEventHelper.BroadcastEvent(DependencyObject root, RoutedEvent routedEvent) in /_/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/BroadcastEventHelper.cs:line 280
   at System.Windows.BroadcastEventHelper.BroadcastLoadedSynchronously(DependencyObject rootDO, Boolean isLoaded) in /_/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/BroadcastEventHelper.cs:line 192
   at System.Windows.BroadcastEventHelper.BroadcastLoadedEvent(Object root) in /_/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/BroadcastEventHelper.cs:line 180
   at System.Windows.Media.MediaContext.FireLoadedPendingCallbacks() in /_/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/MediaContext.cs:line 1993
   at System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks() in /_/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/MediaContext.cs:line 1980
   at System.Windows.Media.MediaContext.RenderMessageHandlerCore(Object resizedCompositionTarget) in /_/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/MediaContext.cs:line 1841
   at System.Windows.Media.MediaContext.RenderMessageHandler(Object resizedCompositionTarget) in /_/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/MediaContext.cs:line 1744
   at System.Windows.Media.MediaContext.Resize(ICompositionTarget resizedCompositionTarget) in /_/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/MediaContext.cs:line 1727
   at System.Windows.Interop.HwndTarget.OnResize() in /_/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/InterOp/HwndTarget.cs:line 1601
   at System.Windows.Interop.HwndTarget.HandleMessage(WindowMessage msg, IntPtr wparam, IntPtr lparam) in /_/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/InterOp/HwndTarget.cs:line 1099
   at System.Windows.Interop.HwndSource.HwndTargetFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled) in /_/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/InterOp/HwndSource.cs:line 1188
   at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled) in /_/src/Microsoft.DotNet.Wpf/src/Shared/MS/Win32/HwndWrapper.cs:line 295
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o) in /_/src/Microsoft.DotNet.Wpf/src/Shared/MS/Win32/HwndSubclass.cs:line 429
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs) in /_/src/Microsoft.DotNet.Wpf/src/WindowsBase/MS/Internal/Threading/ExceptionWrapper.cs:line 103
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler) in /_/src/Microsoft.DotNet.Wpf/src/WindowsBase/MS/Internal/Threading/ExceptionWrapper.cs:line 36
   at System.Windows.Threading.Dispatcher.WrappedInvoke(Delegate callback, Object args, Int32 numArgs, Delegate catchHandler) in /_/src/Microsoft.DotNet.Wpf/src/WindowsBase/System/Windows/Threading/Dispatcher.cs:line 2859
   at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs) in /_/src/Microsoft.DotNet.Wpf/src/WindowsBase/System/Windows/Threading/Dispatcher.cs:line 1331
   at System.Windows.Threading.Dispatcher.Invoke(DispatcherPriority priority, Delegate method, Object arg) in /_/src/Microsoft.DotNet.Wpf/src/WindowsBase/System/Windows/Threading/Dispatcher.cs:line 1050
   at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam) in /_/src/Microsoft.DotNet.Wpf/src/Shared/MS/Win32/HwndSubclass.cs:line 341

Reverting this commit fixed the crash.