Add context-aware read/write (#57)

Adds ReadContext/WriteContext methods to endpoints and to streams. Update rawread example tool to use ReadContext for implementation of "timeout" flag.

endpoint_stream.go

@@ -17,7 +17,7 @@ package gousb
 func (e *endpoint) newStream(size, count int) (*stream, error) {
 	var ts []transferIntf
 	for i := 0; i < count; i++ {
-		t, err := newUSBTransfer(e.ctx, e.h, &e.Desc, size, e.Timeout)
+		t, err := newUSBTransfer(e.ctx, e.h, &e.Desc, size)
 		if err != nil {
 			for _, t := range ts {
 				t.free()
@@ -29,8 +29,8 @@ func (e *endpoint) newStream(size, count int) (*stream, error) {
 	return newStream(ts), nil
 }
 
-// NewStream prepares a new read stream that will keep reading data from the
-// endpoint until closed.
+// NewStream prepares a new read stream that will keep reading data from
+// the endpoint until closed or until an error or timeout is encountered.
 // Size defines a buffer size for a single read transaction and count
 // defines how many transactions should be active at any time.
 // By keeping multiple transfers active at the same time, a Stream reduces
@@ -44,11 +44,11 @@ func (e *InEndpoint) NewStream(size, count int) (*ReadStream, error) {
 	return &ReadStream{s: s}, nil
 }
 
-// NewStream prepares a new write stream that will write data in the background.
-// Size defines a buffer size for a single write transaction and count
-// defines how many transactions may be active at any time.
-// By buffering the writes, a Stream reduces the latency between subsequent
-// transfers and increases writing throughput.
+// NewStream prepares a new write stream that will write data in the
+// background. Size defines a buffer size for a single write transaction and
+// count defines how many transactions may be active at any time. By buffering
+// the writes, a Stream reduces the latency between subsequent transfers and
+// increases writing throughput.
 func (e *OutEndpoint) NewStream(size, count int) (*WriteStream, error) {
 	s, err := e.newStream(size, count)
 	if err != nil {