- added missing constraint check when sub-clocking

   tuple expressions
 - added an algorithm that reuses dead or clock-disjoint 
   variables instead of declaring/using new ones.
   - NOT carefully tested. Use option -O 3 if you want 
     to give it a try
   - some output name may change from node to machine code,
     this point could be discussed...
   - further optimizations of machine code are now enabled
     (eg. merging case constructs)