Use file and directory naming standard

[richelbilderbeek]

Currently, the repo has a mix of naming styles.

• the root folder follows snake_case, except for the README.md
• other folders have uppercase letters in them, e.g. extra_BC

I suggest to use a standarized naming, snake_case, where the folder mentioned would become extra_bc.

I volunteer to do this and fix the broken links.

[richelbilderbeek]

Discussed and accepted:

- [Richel, discuss, 5 mins] Discuss <https://github.com/UPPMAX/programming_formalisms/issues/22>
  - Richel will do this

[richelbilderbeek]

Done!