Defines a WriteStream structure for buffering writes, similar to the existing ReadStream. WriteStreams can be created on OutEndpoints by using ep.NewStream().
* Store a reference to libusb implementation in the context, transfers
and some more places. Remove the global libusb variable.
* Parallelize tests.
* Fix the link in README.
* Add APIs for config and interface descriptors. Split out the common
parts of selecting a config descriptor from device desc and
selecting a setting descriptor from a config desc.
* Parallelize the few tests that actually can be parallelized safely.
Add comments where they can't. Note to self: it would be beneficial
to restructure the fakelibusb to index all properties of the lib
with the used context. That way a libusb implementation wouldn't need
to be referred via a shared variable.
