Rework module system

Don't require/allow naming a module, instead require that the importer
names it. Allow importing into the local scope with the name !Local.
Simplify namespacing logic. Updates all tests to reflect these changes.

test/Spec.hs

@@ -532,6 +532,9 @@ modules = testGroup "Test modules"
   , testCase "Lambda expression namespaces" $ do
       res <- liftIO $ evaluateFileResult "./test/lambda-A.tri"
       res @?= Leaf
+  , testCase "Local namespace import chain" $ do
+      res <- liftIO $ evaluateFileResult "./test/local-ns/1.tri"
+      res @?=  Fork (Stem Leaf) (Fork (Stem Leaf) Leaf)
   ]