Const stub

Related issue: https://github.com/igrep/crispy/issues/1
- Implement Crispy.stub_const.
- Change module structure:
    - Now public modules are prefixed with `Crispy` instead of `Crispy::`.
    - Because it seems that we can't refer with `Crispy::World` when including the top level `::Crispy` module.
    - So now we can refer with `CrispyWorld` by including `::Crispy`