Config and interface description (#16)

* 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.

interface.go

@@ -52,6 +52,8 @@ type InterfaceSetting struct {
 	// Endpoints enumerates the endpoints available on this interface with
 	// this alternate setting.
 	Endpoints map[EndpointAddress]EndpointDesc
+
+	iInterface int // index of a string descriptor describing this interface.
 }
 
 func (a InterfaceSetting) sortedEndpointIds() []string {