Fix vool assignments after call rework

also small fix for if and return, as they need to execute sneds and yields (not just sends), so testing for Call not SendStatement

lib/vool/return_statement.rb

@@ -18,7 +18,7 @@ module Vool
     def to_mom( compiler )
       load = Mom::SlotLoad.new( self , [:message , :return_value] ,
                         @return_value.slot_definition(compiler) )
-      if @return_value.is_a?(SendStatement)
+      if @return_value.is_a?(CallStatement)
         ret = @return_value.to_mom(compiler)
         ret << load
       else