move cryptographicallySecure into Backend type

This is groundwork for external backends, but also makes sense to keep
this information with the rest of a Backend's implementation.

Also, removed isVerifiable. I noticed that the same information is
encoded by whether a Backend implements verifyKeyContent or not.

Types/Key.hs

@@ -23,8 +23,6 @@ module Types.Key (
 	HashSize(..),
 	hasExt,
 	sameExceptExt,
-	cryptographicallySecure,
-	isVerifiable,
 	formatKeyVariety,
 	parseKeyVariety,
 ) where
@@ -262,36 +260,6 @@ sameExceptExt (SHA1Key _) (SHA1Key _) = True
 sameExceptExt (MD5Key _) (MD5Key _) = True
 sameExceptExt _ _ = False
 
-{- Is the Key variety cryptographically secure, such that no two differing
- - file contents can be mapped to the same Key? -}
-cryptographicallySecure :: KeyVariety -> Bool
-cryptographicallySecure (SHA2Key _ _) = True
-cryptographicallySecure (SHA3Key _ _) = True
-cryptographicallySecure (SKEINKey _ _) = True
-cryptographicallySecure (Blake2bKey _ _) = True
-cryptographicallySecure (Blake2bpKey _ _) = True
-cryptographicallySecure (Blake2sKey _ _) = True
-cryptographicallySecure (Blake2spKey _ _) = True
-cryptographicallySecure _ = False
-
-{- Is the Key variety backed by a hash, which allows verifying content?
- - It does not have to be cryptographically secure against eg birthday
- - attacks.
- -}
-isVerifiable :: KeyVariety -> Bool
-isVerifiable (SHA2Key _ _) = True
-isVerifiable (SHA3Key _ _) = True
-isVerifiable (SKEINKey _ _) = True
-isVerifiable (Blake2bKey _ _) = True
-isVerifiable (Blake2bpKey _ _) = True
-isVerifiable (Blake2sKey _ _) = True
-isVerifiable (Blake2spKey _ _) = True
-isVerifiable (SHA1Key _) = True
-isVerifiable (MD5Key _) = True
-isVerifiable WORMKey = False
-isVerifiable URLKey = False
-isVerifiable (OtherKey  _) =  False
-
 formatKeyVariety :: KeyVariety -> S.ByteString
 formatKeyVariety v = case v of
 	SHA2Key sz e -> adde e (addsz sz "SHA")