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/experiment/tlstool/internal/splitter.go

@@ -0,0 +1,67 @@
+package internal
+
+import (
+	"bytes"
+	"math/rand"
+	"time"
+)
+
+// SNISplitter splits input such that SNI is splitted across
+// a bunch of different output buffers.
+func SNISplitter(input []byte, sni []byte) (output [][]byte) {
+	idx := bytes.Index(input, sni)
+	if idx < 0 {
+		output = append(output, input)
+		return
+	}
+	output = append(output, input[:idx])
+	// TODO(bassosimone): splitting every three bytes causes
+	// a bunch of Unicode chatacters (e.g., in Chinese) to be
+	// sent as part of the same segment. Is that OK?
+	const segmentsize = 3
+	var buf []byte
+	for _, chr := range input[idx : idx+len(sni)] {
+		buf = append(buf, chr)
+		if len(buf) == segmentsize {
+			output = append(output, buf)
+			buf = nil
+		}
+	}
+	if len(buf) > 0 {
+		output = append(output, buf)
+		buf = nil
+	}
+	output = append(output, input[idx+len(sni):])
+	return
+}
+
+// Splitter84rest segments the specified buffer into three
+// sub-buffers containing respectively 8 bytes, 4 bytes, and
+// the rest of the buffer. This segment technique has been
+// described by Kevin Bock during the Internet Measurements
+// Village 2020: https://youtu.be/ksojSRFLbBM?t=1140.
+func Splitter84rest(input []byte) (output [][]byte) {
+	if len(input) <= 12 {
+		output = append(output, input)
+		return
+	}
+	output = append(output, input[:8])
+	output = append(output, input[8:12])
+	output = append(output, input[12:])
+	return
+}
+
+// Splitter3264rand splits the specified buffer at a random
+// offset between 32 and 64 bytes. This is the methodology used
+// by github.com/Jigsaw-Code/outline-go-tun2socks.
+func Splitter3264rand(input []byte) (output [][]byte) {
+	if len(input) <= 64 {
+		output = append(output, input)
+		return
+	}
+	rnd := rand.New(rand.NewSource(time.Now().UnixNano()))
+	offset := rnd.Intn(32) + 32
+	output = append(output, input[:offset])
+	output = append(output, input[offset:])
+	return
+}