chore: merge probe-engine into probe-cli (#201)

This is how I did it:

1. `git clone https://github.com/ooni/probe-engine internal/engine`

2. ```
(cd internal/engine && git describe --tags)
v0.23.0
```

3. `nvim go.mod` (merging `go.mod` with `internal/engine/go.mod`

4. `rm -rf internal/.git internal/engine/go.{mod,sum}`

5. `git add internal/engine`

6. `find . -type f -name \*.go -exec sed -i 's@/ooni/probe-engine@/ooni/probe-cli/v3/internal/engine@g' {} \;`

7. `go build ./...` (passes)

8. `go test -race ./...` (temporary failure on RiseupVPN)

9. `go mod tidy`

10. this commit message

Once this piece of work is done, we can build a new version of `ooniprobe` that
is using `internal/engine` directly. We need to do more work to ensure all the
other functionality in `probe-engine` (e.g. making mobile packages) are still WAI.

Part of https://github.com/ooni/probe/issues/1335

internal/engine/netx/trace/saver.go

@@ -0,0 +1,27 @@
+package trace
+
+import "sync"
+
+// The Saver saves a trace
+type Saver struct {
+	ops []Event
+	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)
+}