implement a private state for the uchar.h functions

The C standard is imperative on that:

7.28.1 ... If ps is a null pointer, each function uses its own internal
mbstate_t object instead, which is initialized at program startup to
the initial conversion state;

and these functions are also not supposed to implicitly use the state of
the wchar.h functions:

7.29.6.3 ... The implementation behaves as if no library function calls
these functions with a null pointer for ps.

Previously this resulted in two bugs.

- The functions c16rtomb and mbrtoc16 would crash when called with ps
set to null.

- The function mbrtoc32 used the private state of mbrtowc, which it
is not allowed to do.

implement uchar.h (C11 UTF-16/32 conversion) interfaces