Cleanup + code organization

[MirceaS]

• Move regexp codebase into its own separate folder examples/regex
• Audit theorems to make sure they all have the same format (eg. thm_name_forwards, thm_name_reverse, followed by hm_name (for the bijection))
• Remove double whitespaces from the middle of code

[nishantjr]

How do you feel about theories/theory-name?
This would lead to the organization:

• matching-logic.mm0
• theories/
  • 0-empty/
    • 1-helpers.mm1
  • 1-definedness/
    • 0-axioms.mm0 (for axioms and public notation)
    • 1-helpers.mm1
    • 2-proof-system-p.mm1
    • 3-fixedpoints.mm1
  • 2-words/
    • 0-axioms.mm0
    • ...