Use new Data.Store.Version to address #2296

[mgsloan]

Not quite ready yet, still pending review and release of the changes to store.

Also, the type hashes change when moving to 7.8 due to a type changing location:

--- /home/mgsloan/fpco/stack/.stack-work/store-versioned/aHzcZ6_w3rL6NtEJUqEfh6fcjAc=   2016-08-02 02:54:02.680394461 -0700
+++ /home/mgsloan/fpco/stack/.stack-work/store-versioned/QQs6brImltateKQyGnt1TgdkoVM=   2016-08-02 02:59:26.306286816 -0700
@@ -13,6 +13,13 @@
     slot 0 :: [(Stack.Types.PackageIdentifier.PackageIdentifier,Stack.Types.PackageIndex.PackageCache)]
   }

+data-type Data.Maybe.Maybe Stack.Types.PackageIndex.PackageDownload
+  = Nothing {
+  }
+  | Just {
+    slot 0 :: Stack.Types.PackageIndex.PackageDownload
+  }
+
 data-type Data.Text.Internal.Text
   = pack {
     slot 0 :: [GHC.Types.Char]
@@ -20,13 +27,6 @@

 data-type Data.Vector.Unboxed.Base.Vector GHC.Types.Word ignored

-data-type GHC.Base.Maybe Stack.Types.PackageIndex.PackageDownload
-  = Nothing {
-  }
-  | Just {
-    slot 0 :: Stack.Types.PackageIndex.PackageDownload
-  }
-
 data-type GHC.Int.Int64 has IntRep

 data-type GHC.Types.Char has CharRep
@@ -43,7 +43,7 @@
   = PackageCache {
     pcOffset :: GHC.Int.Int64
     pcSize :: GHC.Int.Int64
-    pcDownload :: GHC.Base.Maybe Stack.Types.PackageIndex.PackageDownload
+    pcDownload :: Data.Maybe.Maybe Stack.Types.PackageIndex.PackageDownload
   }

 data-type Stack.Types.PackageIndex.PackageCacheMap

[borsboom]

Does this merge unblock making a new Stack release?

[mgsloan]

I believe so, yes. One of the nice things about this new Data.Store.Version stuff is it gives full control of the actual tag used for serialization, while still checking the structure hash. This means that we'll have full control of when the binary caches get invalidated.