Switch from delegates to UnmanagedCallersOnly and function pointers

[chkn]

Now that we are targeting .NET 6, we can make use of the UnmanagedCallersOnlyAttribute and function pointers, and remove the overhead of managed delegates for calling to/from unmanaged code.

We will know this work is done when we can remove Delegates.cs.

Here is a (probably incomplete) list of methods to convert to UnmanagedCallersOnlyAttribute:

• CustomViewType.Body
• Button.OnAction and Button.OnDispose
• All the value witness table callbacks in ManagedSwiftType