add new kind of test: generating .h files. and more

* docgen supports obj_err code kind for demonstrating
   errors without explicit test cases
 * add documentation for `extern enum`. See #367
 * remove coldcc keyword and add @setIsCold. See #661
 * add compile errors for non-extern struct, enum, unions
   in function signatures
 * add .h file generation for extern struct, enum, unions