Encode unsigned integer like asn.1 der length
#include <fmt.h>
size_t fmt_asn1derlength(char *dest,unsigned long long source);
fmt_asn1derlength encodes an unsigned integer using the ASN.1 DER for encoding tag lengths. This can take from 1 byte (0-0x7f) up to sizeof(source)+1 bytes.
If dest equals FMT_LEN (i.e. is NULL), fmt_asn1derlength returns the number of bytes it would have written.
For convenience, fmt.h defines the integer FMT_ASN1LENGTH to be big enough to contain every possible fmt_asn1derlength output.