Improvements to source file position tracking.

Be better about tracking the full extent of expressions in the parser;
this leads to more intelligible error messages when we indicate where
exactly the error happened.

ast.h

@@ -69,7 +69,7 @@ public:
 
     /** All AST nodes must track the file position where they are
         defined. */
-    const SourcePos pos;
+    SourcePos pos;
 };