Don't treat 'external' parsers as more experimental. All parsers
depend on crates to some extend, and all have C glue code. So the
distinction doesn't really make sense.
Converting the NTP parser to the new registration method is a simple,
3-steps process:
- change the extern functions to use generic input parameters (functions
in all parsers must share common types to be generic) and cast them
- declare the Parser structure
- remove the C code and call the registration function