Add magic numbers to the serialize/deserialize methods

[adamnovak]

It would be handy to be able to identify the files' types. Serialize would prefix each file with some constant bytes for each graph type, and deserialize would skip over them.

We could also maybe add a method to SerializableHandleGraph that would return the magic number that the class uses, to let larger programs sniff types auto-magically.

VG's IO system now has support for ID-ing non-VPKG files by magic number, so this would help compatibility with vg.

[jeizenga]

We could use one of those _impl schemes for serialize/deserialize to ensure that each implementation prefixes a serialization with its magic number. As in, have a non-virtual serialize and deserializethat take care of the magic number and the virtual serialize_impl/deserialize_impl

[jeizenga]

Proposal:

public:

virtual static uint64_t get_magic_number();

private:

virtual void serialize_impl(ostream& out) const;

virtual void deserialize_impl(istream& in);

public:

void serialize(ostream& out) const {
    out << htonl(get_magic_number());
    serialize_impl(out);
}

void deserialize(istream& in) {
    uint64_t magic_number;
    in.read((char*) &magic_number, sizeof(magic_number) / sizeof(char));
    magic_number = ntohl(magic_number);
    assert(magic_number == get_magic_number());
    deserialize_impl(in);
}