v0.4.0

• Tidy the header files into folders and subfolders
• Use an Implementation / declaration approach on headers, this enables
  • Having a single header source code
  • "include loops" so classes A and B can call each others methods even though both are entirely defined in headers
• Refactor the evaluation code
• Move away from string views into the original expression string and use string copies and a stringview-like class SubstrInfo
  • Had many potential issues with dangling pointers.
• Add the rpn namespace: create and evaluate every math object with the reverse polish notation / postfix representation.
  • This representation has faster evaluation but slower parsing (because it goes through an AST representation first)

Full Changelog: v0.3.0...v0.4.0