make aio::Encoder actually behave with async

make aio::Encoder actually behave with async

To really use the encoder with async/await, it needs to support
SeqWrite implementations that are Send. This requires changing a whole
bunch of '&mut dyn SeqWrite' trait objects to instead take a 'T:
SeqWrite' generic parameter directly instead. Most of this is quite
straightforward, though incurs a lot of churn (FileImpl needs a generic
parameter now for example).

The trickiest part is returning a new Encoder instance in
create_directory, as the trait object trick with
SeqWrite::as_trait_object doesn't work if SeqWrite is implemented for
generic '&mut S'.

Instead, work with the generic object directly, and express the
owned/borrowed state in the Encoder (to avoid nested borrowing) as an
enum EncoderOutput.

Add to the aio test to ensure the Encoder is now actually useable.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>