refactor: move tracex outside of engine/netx (#782)

* refactor: move tracex outside of engine/netx

Consistently with https://github.com/ooni/probe/issues/2121 and
https://github.com/ooni/probe/issues/2115, we can now move tracex
outside of engine/netx. The main reason why this makes sense now
is that the package is now changed significantly from the one
that we imported from ooni/probe-engine.

We have improved its implementation, which had not been touched
significantly for quite some time, and converted it to unit
testing. I will document tomorrow some extra work I'd like to
do with this package but likely could not do $soon.

* go fmt

* regen tutorials

internal/tracex/saver.go

@@ -0,0 +1,35 @@
+package tracex
+
+//
+// Saver implementation
+//
+
+import "sync"
+
+// The Saver saves a trace. The zero value of this type
+// is valid and can be used without initialization.
+type Saver struct {
+	// ops contains the saved events.
+	ops []Event
+
+	// mu provides mutual exclusion.
+	mu sync.Mutex
+}
+
+// Read reads and returns events inside the trace. It advances
+// the read pointer so you won't see such events again.
+func (s *Saver) Read() []Event {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	v := s.ops
+	s.ops = nil
+	return v
+}
+
+// Write adds the given event to the trace. A subsequent call
+// to Read will read this event.
+func (s *Saver) Write(ev Event) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.ops = append(s.ops, ev)
+}