also:
- added `toml_version.h`
- added version number to toml.hpp
- added `node::visit()`
- added `table::empty()`
- added `array::empty()`
- added version number to `toml.hpp`
- added ostream operator overload for `source_position`
- added tests for string escape sequences
- added tests for + in bare keys (toml/issues/687)
- added tests for hexfloat literals (toml/issues/562)
- added c++20 char8_t detection to meson
- moved third party submodules to `/extern`
- refactored noexcept version of `parse_result` to be more useful
- refactored all code to 'mostly' stick to a 120 column limit
- fixed some minor stuff picked up by gcc9
