also:
- fixed a bunch of doxygen parsing issues
- added `#define` leak detection to the single-header script
- renamed `TOML_ALL_INLINE` to `TOML_HEADER_ONLY` (the old one still works too)
- simplified abi namespace definitions
also:
- fixed value comparison with special floats
- added all the remaining conformance tests from BurntSushi/toml-test and iarna/toml-spec-tests
- added toml::inserter
- added license boilerplate to test files