Use Lotus::String#deconstantize to detect application_module

Hi.

On defining following Application.
```
module Hoge
  module V1
    class Application < Lotus::Application
      ...
    end
  end
end
```
Lotus::Loader will load framework to toplevel module(Hoge) by this line
https://github.com/lotus/lotus/blob/41d1fe8624b5f26c111cc019283602fedc8fd237/lib/lotus/loader.rb#L98

This PR make the Loader will load framework into application's own parent module.

It depends on https://github.com/lotus/utils/pull/28