X25519 Support (#3686)

* early days

* sort of working

* more things

* remove private_bytes

* public bytes, interface fix

* load public keys

* x25519 support basically done now

* private_bytes is gone

* some reminders

* doctest this too

* remove a thing that doesn't matter

* x25519 supported checks

* libressl has the NID, but a different API, so check for OpenSSL

* pep8

* add missing coverage

* update to use reasons

* expand test a little

* add changelog entry

* review feedback
7 files changed