Renaming Vool exppressions rightly

Class, Method and Lambda (was block) are expressions.
Just making things clearer, especially for the blocks (ahem, lambdas) is matters.
wip

lib/vool/if_statement.rb

@@ -15,7 +15,7 @@ module Vool
       false_label = Mom::Label.new( self , "false_label_#{object_id.to_s(16)}")
       merge_label = Mom::Label.new( self , "merge_label_#{object_id.to_s(16)}")
 
-      check =  Mom::TruthCheck.new(condition.slot_definition(compiler) , false_label)
+      check =  Mom::TruthCheck.new(condition.to_slot(compiler) , false_label)
       if @condition.is_a?(CallStatement)
         head = @condition.to_mom(compiler)
         head << check