Spring clean the codebase

[mattgodbolt]

Ideally:

• [ ] Minimum C++17
• [ ] use conan (see #40)
• [ ] clang-format with a known good setting
• [ ] sanitizer builds etc
• [ ] General tidying of old fashioned code like:
  • [ ] misuse of move (vs std::move)
  • [ ] use std::string_view
  • [ ] general audit in IDEs that whinge about such things
• [ ] Turn up the warnings and heed them (-Werror for maintainer builds)