 - Mutually recursive types do not lead to non-termination anymore. 
   They are treated properly now.

 - Mutually recursive function definitions supported now. See examples
   mutual0.art, mutual1.art,etc.	
   Syntax: let rec f = <..> and g = <..> and h = <..> in <..>
	
 - Bugfix: Implicit matching of singleton-variant types was
           buggy. This is only syntax-sugar anyway, but it works now.
  	   (one may write "fun A(a,b,c) -> <..>" instead of "fun x -> match x with A(a,b,c) -> <...>" 
	    provided that there is onle one alternative...)
