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.