Rename fd.tt to fd.tcc.
Plug memory leak in tamer::distribute.
Cleanup: Every event<T> has an existent T slot.

In other words, we can no longer have a nonempty event<T> with
null result_pointer().

It is too much of a pain to program with has_result() != empty().

Also rename tamer::unbind() to tamer::rebind().
And add `event<T0> tamer::rebind(event<> e, T0& slot)`.