size_t fmt_longlong(char *dest,long long source);
fmt_longlong writes an ASCII representation ('-' and '0' to '9', base 10) of source to dest and returns the number of bytes written.
fmt_longlong does not append \0.
If dest equals FMT_LEN (i.e. is zero), fmt_longlong returns the number of bytes it would have written.
For convenience, fmt.h defines the integer FMT_LONG to be big enough to contain every possible fmt_longlong output plus \0.