hdl.{ast,dst}: directly represent RTLIL default case.

hdl.{ast,dst}: directly represent RTLIL default case.

This makes RTLIL mildly nicer:

 casez ({ \$5 , \$3 , \$1  })
   3'bzz1:
       \$next\o  = \$7 ;
   3'bz1z:
       \$next\o  = \$9 ;
   3'b1zz:
       \$next\o  = \$11 ;
-  3'bz:
+  default:
       { \$next\co , \$next\o  } = \$13 ;
 endcase