Before this commit the Lwt_stream API provided no reliable way to detect
if a stream had been closed. While on_termination notifies the user if
the stream is closed in the future, it does not tell the user if the
stream was closed in the past.

Introducing the is_closed function addresses this problem, allowing the
user to detect if a stream has been closed at any point in time.

This function returns a thread that will sleep until the stream is
closed. The thread can be used in place of if_closed and on_terminate to
detect when a stream is closed at any point in time.

The hooks will be called at the appropriate time once `f` finally
returns `None`.

As per the discussion on the issue tracker, Lwt_stream.on_termination
will be deprecated and ultimately removed in favor for
Lwt_stream.closed. To facilitate that, this commit introduces a new
implementation of Lwt_stream.closed that does not rely on termination
hooks, and implementation Lwt_stream.on_termination in terms of
Lwt_Stream.closed.

The Lwt_stream module had multiple copies of low-level enqueueing code
of two varieties copied in serveral places.This commit moves one variety
of enqueing code into a single function called enqueue', and implements
the second variety as the function enqueue, in terms of enqueue'.

Previously, the of_list, of_array, and of_string constructors each would
produce a stream s for which

  is_closed s = false

This is counter-intuitive as all the elements of the stream s have been
generated and should be consumable without blocking. This commit changes
the behavior of this constructors such that

  is_closed s = true

immediately upon creation.

This reverts commit 47cd91f.