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.

src/error.rs

@@ -16,6 +16,9 @@ pub enum Error {
     #[fail(display = "{:?}", _0)]
     Message(String),
 
+    #[fail(display = "{:?}", _0)]
+    MessageChain(String, #[cause] failure::Error, failure::Backtrace),
+
     #[fail(display = "{:?}", _0)]
     Image(image_meta::ImageError),
 
@@ -118,6 +121,18 @@ impl From<crate::message::InvalidMsgId> for Error {
     }
 }
 
+impl From<crate::key::SaveKeyError> for Error {
+    fn from(err: crate::key::SaveKeyError) -> Error {
+        Error::MessageChain(format!("{}", err), err.into(), failure::Backtrace::new())
+    }
+}
+
+impl From<crate::pgp::PgpKeygenError> for Error {
+    fn from(err: crate::pgp::PgpKeygenError) -> Error {
+        Error::MessageChain(format!("{}", err), err.into(), failure::Backtrace::new())
+    }
+}
+
 impl From<mailparse::MailParseError> for Error {
     fn from(err: mailparse::MailParseError) -> Error {
         Error::MailParseError(err)