Don't create untracked content in http-parser.
Improve name (based on auto_ptr::release).
Remove unused.
Add tamer::destroy_guard.

Declare a destroy_guard in your tvars if you want the closure to exit
when a tamed_class is deleted.

Unlike tamer::exit_at_destroy, tamer::destroy_guard can link a closure
to more than one tamed_class.

And add a test.