* Create an `src/3rdparty` folder for third-party code (to add the
Grisu2B alfogithm soon).
* Split the formatted printer into gint's kprint (src/kprint), its
extension and interface (include/gint/kprint.h), and its use in the
standard stdio functions (src/std/print.c).
* Slightly improve the interface of kformat_geometry() to avoid relying
on knowing format specifiers.
* Add a function to register more formatters, to allow floating-point
formatters without requiring them.
