Write an ascii representation of an unsigned integer
#include <fmt.h>
size_t fmt_uint(char *dest,unsigned int source);
fmt_uint writes an ASCII representation ('0' to '9', base 10) of source to dest and returns the number of bytes written.
fmt_uint does not append \0.
If dest equals FMT_LEN (i.e. is zero), fmt_uint returns the number of bytes it would have written.
For convenience, fmt.h defines the integer FMT_ULONG to be big enough to contain every possible fmt_uint output plus \0.