Remove From<Path> for Certificate

Deserialization from a path is a heavy conversion. This should be done by
an explicit `Certificate::new_from_cbor` call.

coffer-common/src/certificate.rs

@@ -106,10 +106,3 @@ impl Certificate {
         Ok(sealedbox::seal(message, pk))
     }
 }
-
-impl <T: AsRef<Path>> From<T> for Certificate {
-    fn from(path: T) -> Self {
-        Certificate::new_from_cbor(&path)
-            .expect(&format!{"Could not read certificate from {}", path.as_ref().display()})
-    }
-}

coffer-common/src/keyring.rs

@@ -53,7 +53,7 @@ impl Keyring {
     where T: AsRef<Path>
     {
         Keyring {
-            certificate: Certificate::from(certificate_path),
+            certificate: Certificate::new_from_cbor(certificate_path).unwrap(),
             known_keys: HashMap::new()
         }
     }