openpgp: Rework KeyAmalgamation to preserve the key's role.

  - Introduce three KeyAmalgamation variants:
    `PrimaryKeyAmalgamation`, `SubordinateKeyAmalgamation`, and
    `ErasedKeyAmalgamation`.

  - Unlike a `Key` or a `KeyBundle` with an `UnspecifiedRole`, an
    `ErasedKeyAmalgamation` remembers its role.  This means that an
    `ErasedKeyAmalgamation` can implement the correct semantics even
    though the role marker has been erased (hence the name).

  - Have `Cert::keys` return `ErasedKeyAmalgamation`s.  Recall:
    `Cert::keys` can't return a more specific type, because it returns
    an iterator that can contain both primary and subordinate keys.

    - We use a concrete type instead of a trait object so that when
      the user converts a `KeyAmalgamation` to a
      `ValidKeyAmalgamation` (via `with_policy`), the
      `ValidKeyAmalgamation` retains the type information about the
      `KeyAmalgamation`'s role.

    - Preserving this type information increases type safety for users
      of this API.

1 files changed, 0 insertions, 1 deletions

diff --git a/examples/guide-exploring-openpgp.rs b/examples/guide-exploring-openpgp.rs
index 2263dfe0..298fcd6c 100644
--- a/examples/guide-exploring-openpgp.rs
+++ b/examples/guide-exploring-openpgp.rs
@@ -1,7 +1,6 @@
 //! https://sequoia-pgp.org/guide/exploring-openpgp/
 
 extern crate sequoia_openpgp as openpgp;
-use crate::openpgp::cert::prelude::*;
 use crate::openpgp::parse::Parse;
 use crate::openpgp::policy::StandardPolicy as P;