Way to finish compression without closing/shutdown of the underlying `AsyncWrite`

[jakubdabek]

Right now there is only into_inner which doesn't finalize the en/decoder's job, and close/shutdown passes to the writer it is wrapping, which is sometimes undesirable.

[Majored]

@Nemo157 I assume you wouldn't want to change the semantics of the current into_inner() implementations, so do you have any preference for the naming of a new one?

into_inner_finish() finish_and_into_inner() etc.