Clear Up Fillers

[kangrongji]

This PR reorganizes the cube fillers in library. They've been renamed uniformly, deduced as corollaries of the extend operation and now collected in Cubical.Foundations.HLevels.Fillers. There are still some left in Cubical.Foundations.Prelude due to their preliminary nature. @mortberg

To achieve this, I have to separate the basic definitions and facts about h-levels from Cubical.Foundations.HLevels into a new file Cubical.Foundations.HLevels.Base. Additionally, I separated the cube types in Cubical.Foundations.Prelude to the file Cubical.Foundations.Cubes.