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/Backend.hs

@@ -2,7 +2,7 @@
  -
  - Most things should not need this, using Types instead
  -
- - Copyright 2010-2019 Joey Hess <id@joeyh.name>
+ - Copyright 2010-2020 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -18,7 +18,8 @@ import Utility.FileSystemEncoding
 data BackendA a = Backend
 	{ backendVariety :: KeyVariety
 	, getKey :: Maybe (KeySource -> MeterUpdate -> a Key)
-	-- Verifies the content of a key.
+	-- Verifies the content of a key using a hash. This does not need
+	-- to be cryptographically secure.
 	, verifyKeyContent :: Maybe (Key -> FilePath -> a Bool)
 	-- Checks if a key can be upgraded to a better form.
 	, canUpgradeKey :: Maybe (Key -> Bool)
@@ -28,6 +29,8 @@ data BackendA a = Backend
 	-- Checks if a key is known (or assumed) to always refer to the
 	-- same data.
 	, isStableKey :: Key -> Bool
+	-- Checks if a key is verified using a cryptographically secure hash.
+	, isCryptographicallySecure :: Key -> Bool
 	}
 
 instance Show (BackendA a) where