It does not need to hold an anonymous proc for constant search.
Also, this change can be expected to cause an anonymous proc to be GC'd.

This is useful for metaprogramming that makes heavy use of the `class`/`module`/`def` syntax in the `class_eval`/`eval` method.

Example:

- code

  ```ruby
  p ObjectSpace.count_objects

  String.class_eval do
    def a
    end
  end

  p ObjectSpace.count_objects

  String.class_eval do
    eval <<~CODE
      def b
      end
    CODE
  end

  p ObjectSpace.count_objects
  ```

- result of building mruby-head (d63c0df6) with `build_config/default.rb`

  ```
  {:TOTAL=>1024, :FREE=>262, :T_PROC=>495, :T_ENV=>61, ...}
  {:TOTAL=>1024, :FREE=>259, :T_PROC=>497, :T_ENV=>62, ...}
  {:TOTAL=>1024, :FREE=>255, :T_PROC=>500, :T_ENV=>63, ...}
  ```

- result of building mruby with this patch and `build_config/default.rb`

  ```
  {:TOTAL=>1024, :FREE=>264, :T_PROC=>494, :T_ENV=>60, ...}
  {:TOTAL=>1024, :FREE=>262, :T_PROC=>495, :T_ENV=>61, ...}
  {:TOTAL=>1024, :FREE=>261, :T_PROC=>496, :T_ENV=>61, ...}
  ```