Turn `StorePathMetadata` into `Metadata a`

[sorki]

Related to #146, allows us to extend various path types, motivation in #148.

Draft:

 -- | Metadata about a 'StorePath'
-data StorePathMetadata = StorePathMetadata
+data Metadata a = Metadata
   { -- | The path this metadata is about
-    path :: !StorePath
+    path :: !a
   , -- | The path to the derivation file that built this path, if any
     -- and known.
-    deriverPath :: !(Maybe StorePath)
+    deriverPath :: !(Maybe a)
   , -- TODO should this be optional?
     -- | The hash of the nar serialization of the path.
     narHash :: !SomeNamedDigest
   , -- | The paths that this path directly references
-    references :: !StorePathSet
+    references :: !(HashSet a)
   , -- | When was this path registered valid in the store?
     registrationTime :: !UTCTime
   , -- | The size of the nar serialization of the path, in bytes.
@@ -43,6 +43,8 @@ data StorePathMetadata = StorePathMetadata
     contentAddressableAddress :: !(Maybe ContentAddressableAddress)
   }
 
+type StorePathMetadata = Metadata StorePath

[Ericson2314]

In my https://github.com/obsidiansystems/hnix-store/tree/daemon-server-side and in C++ Nix, I just got rid of the field. That, for example, normalizes (in the DB sense) Map StorePath Metadata.