Ada case-statement

[Stephen Davies <joviangm@gmail.com>]

I guess that this has probably been considered before,
but it would be nice if Ada allowed sub-cases, with
coverage checking of the appropriate alternatives.
This seems like a simpler change than some other recent
ones (e.g. conditional expressions), and would remove
one problem with not having non-contiguous subtypes.
For example:

   case Calculate_Day is
      when Mon .. Fri =>
         ...
      when Weekend : Sat | Sun =>
         ...
         case Weekend is
            when Sat =>
               ...
            when Sun =>
               ...
         end case;
   end case;

Ok, I know this isn't going to happen, I just wanted to
put it out there anyway.