also:
- added `date_time` converting constructors from `date` and `time`
- added `is_key<>` and is_key_or_convertible<>` metafunctions
- exposed `TOML_NAMESPACE_START` and `TOML_NAMESPACE_END` macros to help with ADL specialization scenarios
- added encoder and decoder for `toml-test` (closes#125)
- renamed options to `snake_case`
- tests, examples and cmake config now explicitly disabled when used as a subproject
- removed small_binaries (it's now implicit when building as release)
- added minimum meson version 0.53