Add debug representation for `MarkerTree`

[ibraheemdev]

Summary

Adds a simple Debug representation for MarkerTree based on the underlying decision diagram. For example, python_version == '3.7' and os_name == 'Linux') or os_name == 'Windows' is displayed as:

python_version<3.7 | >3.7 -> 
  os_name<Windows | >Windows -> false
  os_name==Windows -> true
python_version==3.7 -> 
  os_name<Linux | >Linux, <Windows | >Windows -> false
  os_name==Linux | ==Windows -> true

I'm not sure we want to use this form for our debug logs because it could be confusing to users but we've also run into issues where markers looked equal when simplified despite being represented slightly differently..

[ibraheemdev]

We should probably add a separate method display_maybe_empty for user facing output.

[BurntSushi]

but we've also run into issues where markers looked equal when simplified despite being represented slightly differently..

Can you show an example? I really like how our Debug implementation works today, especially since it fits on one line. (Although that does break down for larger markers.)