Purify file operations

Factor out storeFile, unstoreFile, and verifyFile into a MTL
style class. This class can provide its own Id type, and never
returns FilePaths directly, which should allow a memory-only
variant for testing purposes later.

Control of storage ids now belongs to MonadFileStore. It was not
really the place of the data layer (which is currently a set of
SQL queries) to suddenly do one arbitrary file IO operation.

package.yaml

@@ -13,6 +13,7 @@ default-extensions:
 - DerivingVia
 - ScopedTypeVariables
 - TypeApplications
+- TypeFamilies
 
 ghc-options:
 - -Wall
@@ -45,10 +46,12 @@ library:
   - optparse-applicative
   - recursion-schemes
   - sqlite-simple
+  - transformers
   exposed-modules:
   - CAStore.Command
   - CAStore.Command.Autocomplete
   - CAStore.Command.Type
+  - CAStore.FileStore
   - CAStore.Config
   - CAStore.Config.CLI
   - CAStore.Config.Type