Code Reorg for EH UnivMon optimization

- Rename enum in chapter.rs
- `struct EHSketchBucket` and `struct EHVolume` are highly duplicated
- `struct EHUnivOptimized` may share "something" with a regular EH+UnivMon
- Reorg EH related files into a subdir, and re-export
- Limit new trait
  - avoid situation that EH, as a functionality, has many traits