Mainly to simplify a the error handling code (it had gotten a bit verbose), but also to reduce compiled binary sizes.
also:
- moved windows terminal code page stuff in examples to a separate file
- added doxygen page for the configuration options
- added SPDX-License-Identifiers around the place
- changed TOML_UNRELEASED_FEATURES default to 0
- simplified ABI namespace machinery
- fixed a number of doxygen bugs