Force Aead::AeadImpl to be Send + Sync (#61)

When trying to use AeadCtxR/AeadCtxS in a generic context which requires
Send or Sync, downstream users need to add a bound to Aead::AeadImpl
even though this is a doc(hidden) item and thus an implementation detail
that may change afterwards.

All implementations of Aead provide an AeadImpl type that is already
Send + Sync so it doesn't hurt to add a trait bound there.

src/aead.rs

@@ -19,7 +19,7 @@ use zeroize::Zeroize;
 pub trait Aead {
     /// The underlying AEAD implementation
     #[doc(hidden)]
-    type AeadImpl: BaseAeadCore + BaseAeadInPlace + BaseKeyInit + Clone;
+    type AeadImpl: BaseAeadCore + BaseAeadInPlace + BaseKeyInit + Clone + Send + Sync;
 
     /// The algorithm identifier for an AEAD implementation
     const AEAD_ID: u16;