Refactor keypair handling and expose storing keypairs on ffi

The user-visible change here is that it allows the FFI API to save
keys in the database for a context.  This is primarily intended for
testing purposes as it allows you to get a key without having to
generate it.

Internally the most important change is to start using the
SignedPublicKey and SignedPrivateKey types from rpgp instead of
wrapping them into a single Key object.  This allows APIs to be
specific about which they want instead of having to do runtime checks
like .is_public() or so.  This means some of the functionality of the
Key impl now needs to be a trait.

A thid API change is to introduce the KeyPair struct, which binds
together the email address, public and private key for a keypair.

All these changes result in a bunch of cleanups, though more more
should be done to completely replace the Key type with the
SignedPublicKye/SignedPrivateKey + traits.  But this change is large
enough already.

Testing-wise this adds two new keys which can be loaded from disk and
and avoids a few more key-generating tests.  The encrypt/decrypt tests
are moved from the stress tests into the pgp tests and split up.

python/src/deltachat/account.py

@@ -118,6 +118,18 @@ class Account(object):
         assert res != ffi.NULL, "config value not found for: {!r}".format(name)
         return from_dc_charpointer(res)
 
+    def _save_self_keypair(self, addr, public, secret):
+        """See _dc_save_self_keypair() in deltachat.h.
+
+        In other words, you don't need this.
+        """
+        res = lib._dc_save_self_keypair(self._dc_context,
+                                        as_dc_charpointer(addr),
+                                        as_dc_charpointer(public),
+                                        as_dc_charpointer(secret))
+        if res == 0:
+            raise Exception("Failed to set key")
+
     def configure(self, **kwargs):
         """ set config values and configure this account.