cpp: Make signatures belonging to the same user ID sortable

lang/cpp/src/key.h, lang/cpp/src/key.cpp (UserID::Signature::operator<):
New.

lang/cpp/src/key.cpp (signature_index): New.

operator< defines a canonical sort order for signatures belonging to
the same user ID. It is based on the sort order defined by
cmp_signodes() in g10/keylist.c of gnupg. In particular, the ordering
of signatures made with the same key by creation time allows to see
easily whether the most recent signature is a certification or a
revocation.

recent

• GnuPG-bug-id: T5094