Fix for loops

lib/ahem/ast/for_loop.rb

     @iterable.execute(env).each do |i|
       _env = Environment.new
       _env.set(@iterator.name, i)
-      @block.execute(_env)
+      @block.statements.each do |statement|
+        statement.execute(_env)
+      end
     end
   end
 end

spec/ast/for_loop_spec.rb

             AST::Number.new(3.0),
           ]
         ),
-        AST::FunctionCall.new(
-          AST::Identifier.new('print'),
+        AST::Block.new(
           [
-            AST::Identifier.new('x')
+            AST::FunctionCall.new(
+              AST::Identifier.new('print'),
+              [
+                AST::Identifier.new('x')
+              ]
+            )
           ]
         )
       ).execute(Environment.new)