Add write_iter and endpoint_with_additional_data methods

[Windfisch]

Hi,

USB Audio Class devices (including MIDI devices) extend the endpoint descriptor by two additional bytes, bRefresh and bSynchAddress (c.f. the USB MIDI 1.0 spec on page 25/26).

Currently, usb-device offers no way to create such an endpoint. I've added such a function and also a more generic way to write() into the descriptor: write_iter accepts an iterator instead of a slice, allowing for more flexibility using e.g. chained iterators.

I would be happy if this could be merged as it should not cause any incompatibilities.

It would be possible to replace the endpoint() method's implementation body by just a call to endpoint_with_additional_data() with an empty iterator at a tiny runtime cost.

If you prefer to have this change as well or have anything else to discuss, please let me know!

[JosefUtbult]

@Windfisch Hi there. I've been working on trying to get an asynchronous endpoint to work for my soundcard, but I ran into the same problem as you with the lack of fields in the endpoint descriptor. I've been looking through your pull request, but I have a hard time understanding how it should be applied when creating an endpoint. Would you mind helping me? Thanks in advance