openpgp: Use Container for Literal, Unknown.

  - Embed Container in Literal and Unknown.  This reuses code.

  - More importantly, Literal and Unknown now correctly implement
    PartialEq.

  - Unknown cannot implement Ord.  Remove PartialOrd as well, it seems
    like an obscure use case to order packets.

  - An unfortunate consequence of Unknown not implementing Eq is that
    Cert cannot, because it holds unknown packets.  We consider that
    unacceptable.

  - See #93.

1 files changed, 10 insertions, 4 deletions

diff --git a/openpgp/src/packet_pile.rs b/openpgp/src/packet_pile.rs
index dff0411b..f3b3d9c9 100644
--- a/openpgp/src/packet_pile.rs
+++ b/openpgp/src/packet_pile.rs
@@ -244,10 +244,16 @@ impl PacketPile {
                 return Err(Error::IndexOutOfRange.into());
             }
 
-            container = match tmp.packets[i].container_mut() {
-                Some(c) => c,
-                None => return Err(Error::IndexOutOfRange.into()),
-            };
+            match tmp.packets[i] {
+                // The structured container types.
+                Packet::CompressedData(_)
+                    | Packet::SEIP(_)
+                    | Packet::AED(_)
+                    => (), // Ok.
+                _ => return Err(Error::IndexOutOfRange.into()),
+            }
+            container = tmp.packets[i].container_mut()
+                .expect("The above packets are structured containers");
         }
 
         return Err(Error::IndexOutOfRange.into());