Implement timer interface

[senier]

This interface allows users to request the current monotonic time (similar to Clock in Ada.Real_Time). We should also think about function to retrieve precision and bounds of the timer.

[jklmnn]

Proposed timer client interface:

package Cai.Timer is

   type Time is new Duration;

   function "+" (Left : Time; Right : Duration) return Time;
   function "-" (Left : Time; Right : Duration) return Time;
   function "-" (Left : Time; Right : Time) return Duration;
   
   type Client_Session is limited private;

end Cai.Timer

with Cai.Types;

package Cai.Timer.Client is

   function Create return Client_Session;

   function Initialized (C : Client_Session) return Boolean;

   procedure Initialize (C : in out Client_Session; Cap : Cai.Types.Capability);

   function Clock (C : Client_Session) return Time with
      Volatile_Function;

   procedure Finalize (C : in out Client);

end Cai.Timer.Client;

[jklmnn]

Fixed by #54.