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/bytecounter/bytecounter.go

@@ -0,0 +1,54 @@
+package bytecounter
+
+import "github.com/ooni/probe-cli/v3/internal/engine/atomicx"
+
+// Counter counts bytes sent and received.
+type Counter struct {
+	Received *atomicx.Int64
+	Sent     *atomicx.Int64
+}
+
+// New creates a new Counter.
+func New() *Counter {
+	return &Counter{Received: atomicx.NewInt64(), Sent: atomicx.NewInt64()}
+}
+
+// CountBytesSent adds count to the bytes sent counter.
+func (c *Counter) CountBytesSent(count int) {
+	c.Sent.Add(int64(count))
+}
+
+// CountKibiBytesSent adds 1024*count to the bytes sent counter.
+func (c *Counter) CountKibiBytesSent(count float64) {
+	c.Sent.Add(int64(1024 * count))
+}
+
+// BytesSent returns the bytes sent so far.
+func (c *Counter) BytesSent() int64 {
+	return c.Sent.Load()
+}
+
+// KibiBytesSent returns the KiB sent so far.
+func (c *Counter) KibiBytesSent() float64 {
+	return float64(c.BytesSent()) / 1024
+}
+
+// CountBytesReceived adds count to the bytes received counter.
+func (c *Counter) CountBytesReceived(count int) {
+	c.Received.Add(int64(count))
+}
+
+// CountKibiBytesReceived adds 1024*count to the bytes received counter.
+func (c *Counter) CountKibiBytesReceived(count float64) {
+	c.Received.Add(int64(1024 * count))
+}
+
+// BytesReceived returns the bytes received so far.
+func (c *Counter) BytesReceived() int64 {
+	return c.Received.Load()
+}
+
+// KibiBytesReceived returns the KiB received so far.
+func (c *Counter) KibiBytesReceived() float64 {
+	return float64(c.BytesReceived()) / 1024
+}

internal/engine/netx/bytecounter/bytecounter_test.go

@@ -0,0 +1,27 @@
+package bytecounter_test
+
+import (
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/bytecounter"
+)
+
+func TestGood(t *testing.T) {
+	counter := bytecounter.New()
+	counter.CountBytesReceived(16384)
+	counter.CountKibiBytesReceived(10)
+	counter.CountBytesSent(2048)
+	counter.CountKibiBytesSent(10)
+	if counter.BytesSent() != 12288 {
+		t.Fatal("invalid bytes sent")
+	}
+	if counter.BytesReceived() != 26624 {
+		t.Fatal("invalid bytes received")
+	}
+	if v := counter.KibiBytesSent(); v < 11.9 || v > 12.1 {
+		t.Fatal("invalid kibibytes sent")
+	}
+	if v := counter.KibiBytesReceived(); v < 25.9 || v > 26.1 {
+		t.Fatal("invalid kibibytes received")
+	}
+}