[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 {

fine/tests/expression/variable.fine

@@ -7,7 +7,10 @@
 // |     LiteralExpression
 // |       Number:'"23"'
 // |     Semicolon:'";"'
-// |   ExpressionStatement
+// |   LetStatement
+// |     Let:'"let"'
+// |     Identifier:'"y"'
+// |     Equal:'"="'
 // |     BinaryExpression
 // |       Identifier
 // |         Identifier:'"x"'
@@ -15,9 +18,14 @@
 // |       LiteralExpression
 // |         Number:'"2"'
 // |     Semicolon:'";"'
+// |   ExpressionStatement
+// |     Identifier
+// |       Identifier:'"y"'
+// |     Semicolon:'";"'
 // |
 
 let x = 23;
-x * 2;
+let y = x * 2;
+y;
 
-// @type: 416 f64
+// @type: 590 f64