Add -Werror, fix unused import warnings

- Add -Werror to ghc-options for both library and test suite
- Remove unused Data.Maybe import from Internal.hs
- Remove unused Data.Int and Data.Text imports from test files

test/Test/AutoMigration.hs

@@ -7,7 +7,6 @@ module Test.AutoMigration where
 
 import Control.Exception (SomeException, try)
 import Control.Monad.IO.Class (liftIO)
-import Data.Int (Int64)
 import Test.Hspec
 
 import Orville.SQLite

test/Test/EntityOperations.hs

@@ -4,8 +4,6 @@
 module Test.EntityOperations where
 
 import Control.Monad.IO.Class (liftIO)
-import Data.Int (Int64)
-import Data.Text (Text)
 import Test.Hspec
 
 import Orville.SQLite

test/Test/SqlMarshaller.hs

@@ -4,7 +4,6 @@
 
 module Test.SqlMarshaller where
 
-import Data.Int (Int64)
 import qualified Data.Text as T
 import Test.Hspec