refactor if_statement

lib/typed/ast/if_statement.rb

@@ -1,5 +1,7 @@
 module Typed
-  class IfStatement < Statement
-    attr_accessor :branch_type , :condition , :if_true , :if_false
+  module Tree
+    class IfStatement < Statement
+      attr_accessor :branch_type , :condition , :if_true , :if_false
+    end
   end
 end

lib/typed/ast/to_code.rb

@@ -62,7 +62,7 @@ module Typed
 
     def on_if_statement statement
       branch_type , condition , if_true , if_false = *statement
-      w = IfStatement.new()
+      w = Tree::IfStatement.new()
       w.branch_type = branch_type
       w.condition = process(condition)
       w.if_true = process(if_true)