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.

transfer_test.go

@@ -20,6 +20,7 @@ import (
 )
 
 func TestNewTransfer(t *testing.T) {
+	// Can't be parallelized, newFakeLibusb modifies a shared global state.
 	_, done := newFakeLibusb()
 	defer done()
 
@@ -70,6 +71,7 @@ func TestNewTransfer(t *testing.T) {
 }
 
 func TestTransferProtocol(t *testing.T) {
+	// Can't be parallelized, newFakeLibusb modifies a shared global state.
 	f, done := newFakeLibusb()
 	defer done()