Spring clean the codebase

[Matt Godbolt]

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)