Encode unsigned integer like asn.1 der tag
#include <fmt.h>
size_t fmt_asn1dertag(char *dest,unsigned long long source);
fmt_asn1dertag encodes an unsigned integer using the ASN.1 DER for encoding tag. This takes one byte for every 7 bits in the number.
If dest equals FMT_LEN (i.e. is NULL), fmt_asn1dertag returns the number of bytes it would have written.
For convenience, fmt.h defines the integer FMT_ASN1TAG to be big enough to contain every possible fmt_asn1dertag output.