Make Ada.Real_Time.Timing_Events compatible with non-global protected procedures

[liampwll]

Currently it is impossible to use Ada.Real_Time.Timing_Events with a protected object inside a procedure, generic package, etc. due to Timing_Event_Handler having a global accessibility level. There is however an easy fix for this: Optionally allow the handler to be defined as a discriminant on Timing_Event, or rather on a new type to maintain compatibility with any existing code. i.e. type New_Timing_Event (Handler : access protected procedure (Event : Timing_Event_Access)) is tagged limited private;.

One could of course implement this themselves, but since Timing_Events is already here I think it makes sense to have a more accessibility-friendly version.

Another option would be to have an exact copy of Ada.Real_Time.Timing_Events as a generic package, but this is less flexible in a few ways, notably if a protected type wants to hold a timing event as a component which it uses to refer to itself.

[liampwll]

Here is a prototype of the idea. This is a particularly inefficient implementation, however it gives something to play around with: https://gist.github.com/liampwll/a5007b205c7a284ffc07043bfec0d1d9

Edit: The above prototype violates 4.8(5.3) but GNAT incorrectly accepts it. With other compilers or future GNAT versions an Unchecked_Access might be needed here, but for now it still serves its purpose.