Please make hop more complete

Currently, hop only implements a subset of SOP. In order for it to be more universally useful, and to be able to put it into the OpenPGP interoperability test suite, please do implement the remaining functions.

Also, SOP evolved a bit, adding support for inline-signed OpenPGP messages: https://dkg.gitlab.io/openpgp-stateless-cli

And, we're starting to see language-specific interfaces modeled after SOP: https://dkg.gitlab.io/openpgp-stateless-cli/#name-alternate-interfaces

Maybe that is something that would be useful for hOpenPGP too? As an added benefit, once one has that kind of interface, the SOP command-line interface is usually just a thin layer on top, so one gets one interface "for free".