Add methods to make descriptor writing more flexible

[mvirkkunen]

Adds a method for writing descriptors with an unknown length in a way that's similar to how accepting control transfers works, and a method for writing endpoint descriptors with extra data in the same manner.

Unfortunately the naming is the reverse of control transfers (write_with takes a callback while accept_with takes a buffer), but this way it's compatible.

This would implement both #63 and #46. @Windfisch, if you have an audio driver going, could you test this to see if it's good for your purposes?

[Windfisch]

Hey @mvirkkunen, thank you for your effort!
This PR works fine for me (though I still need to manually merge in #62 to support larger descriptors ;)).
I also think that the interface is nice to use, so I'm fine :).

[Windfisch]

@mvirkkunen care to click "merge" on this?