core: add custom Show instance for StorePath

before
```
StorePath {storePathHash = StorePathHashPart {unStorePathHashPart = "#F\139i\215:t\134x\151+\206\&8O*\165\STX\DC2\169\234"}, storePathName = StorePathName {unStorePathName = "testFixed"}}
```

after
```
StorePath /iali40m5597kikibjxw8cx1ssxlqnii3-testFixed
```

Author: sorki

hnix-store-core/src/System/Nix/StorePath.hs

@@ -70,12 +70,18 @@ data StorePath = StorePath
     -- hello-1.2.3).
     storePathName :: !StorePathName
   }
-  deriving (Eq, Generic, Ord, Show)
+  deriving (Eq, Generic, Ord)
 
 instance Hashable StorePath where
   hashWithSalt s StorePath{..} =
     s `hashWithSalt` storePathHash `hashWithSalt` storePathName
 
+instance Show StorePath where
+  show s =
+    "StorePath"
+    <> " "
+    <> storePathToFilePath (StoreDir mempty) s
+
 -- | The name portion of a Nix path.
 --
 -- 'unStorePathName' must only contain a-zA-Z0-9+._?=-, can't start