Add doctests.hs where the cabal file expects it

test/Tests.hs

@@ -1,4 +0,0 @@
-module Main where
-
-main :: IO ()
-main = pure ()

test/doctests.hs

@@ -0,0 +1,10 @@
+import Test.DocTest
+
+main :: IO ()
+main = doctest
+  [ "-isrc"
+  , "src/Lets/GetSetLens.hs"
+  , "src/Lets/Lens.hs"
+  , "src/Lets/OpticPolyLens.hs"
+  , "src/Lets/StoreLens.hs"
+  ]