Add experimental (and undocumented for now) export syntax.

This allows adding types to the list that are included in the automatically-generated
header files.

struct Foo { . . . };
struct Bar { . . . };

export { Foo, Bar };

module.h

@@ -80,6 +80,11 @@ public:
     void AddFunctionDefinition(const std::string &name,
                                const FunctionType *ftype, Stmt *code);
 
+    /** Adds the given type to the set of types that have their definitions
+        included in automatically generated header files. */
+    void AddExportedTypes(const std::vector<std::pair<const Type *, 
+                                                      SourcePos> > &types);
+
     /** After a source file has been compiled, output can be generated in a
         number of different formats. */
     enum OutputType { Asm,      /** Generate text assembly language output */
@@ -145,6 +150,8 @@ private:
     const char *filename;
     AST *ast;
 
+    std::vector<std::pair<const Type *, SourcePos> > exportedTypes;
+
     /** Write the corresponding output type to the given file.  Returns
         true on success, false if there has been an error.  The given
         filename may be NULL, indicating that output should go to standard