[fine] Fixup let environment

Also handle circular references in types and environments without
exploding, and tweak test output a little bit.

fine/tests/example_tests.rs

@@ -107,6 +107,7 @@ fn report_semantic_error(semantics: &Semantics, tr: Option<TreeRef>, message: &s
     }
 
     if let Some(tr) = tr {
+        println!("About the tree: {:?}", &tree[tr]);
         println!("The logical parent chain of the tree was:\n");
         let mut current = Some(tr);
         while let Some(c) = current {