kem: change associated Error bounds to core::error::Error (#2139)

Now that the `Error` trait is stable in `core`, we can bound on it
instead of on `Debug` (where `Error` has a supertrait bound on `Debug`)

Author: tarcieri

kem/src/lib.rs

@@ -10,15 +10,14 @@
 
 pub use crypto_common::{KeyInit, KeySizeUser, typenum::consts};
 
-use core::fmt::Debug;
 use rand_core::TryCryptoRng;
 
 /// A value that can be encapsulated to. Often, this will just be a public key. However, it can
 /// also be a bundle of public keys, or it can include a sender's private key for authenticated
 /// encapsulation.
 pub trait Encapsulate<EK, SS> {
     /// Encapsulation error
-    type Error: Debug;
+    type Error: core::error::Error;
 
     /// Encapsulates a fresh shared secret
     fn encapsulate<R: TryCryptoRng + ?Sized>(&self, rng: &mut R) -> Result<(EK, SS), Self::Error>;
@@ -33,7 +32,7 @@ pub trait Decapsulate<EK, SS> {
     type Encapsulator: Encapsulate<EK, SS>;
 
     /// Decapsulation error
-    type Error: Debug;
+    type Error: core::error::Error;
 
     /// Decapsulates the given encapsulated key
     fn decapsulate(&self, encapsulated_key: &EK) -> Result<SS, Self::Error>;