fmt_utf8 - encode 31-bit unsigned integer using UTF-8 rules

       fmt_utf8  encodes a 31-bit unsigned integer using the UTF-8 rules.  This can take from 1 byte (0-0x7f) up
       to 5 bytes (0x4000000-0x7fffffff).  Values larger than 0x7fffffff cannot be represented in this encoding.

       If dest equals FMT_LEN (i.e. is NULL), fmt_utf8 returns the number of bytes it would have written.

       For convenience, fmt.h defines the integer FMT_UTF8 to be big enough to contain every  possible  fmt_utf8
       output.

       fmt_utf8 - encode 31-bit unsigned integer using UTF-8 rules

       fmt_utf8 and scan_utf8 implement the encoding from UTF-8, but are meant to be able to store integers, not
       just  Unicode  code  points.   Values  larger than 0x10ffff are not valid UTF-8 (see RFC 3629) but can be
       represented in the encoding, so fmt_utf8 will allow them.

scan_utf8(3)

                                                                                                     fmt_utf8(3)

#include<libowfat/fmt.h>

       size_t fmt_utf8(char *dest,uint32_t source);