Initial commit: stalwart-migrator design and scaffolding

In-place upgrade tool for Stalwart Mail Server (0.15.5 -> latest) with
checkpointed rollback and post-migration validation. Design stage; see
ARCHITECTURE.md.

internal/backup/hash.go

@@ -0,0 +1,25 @@
+package backup
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"io"
+	"os"
+)
+
+// hashFile returns a file's SHA256 and size, for recording as a
+// checkpoint.Artifact.
+func hashFile(path string) (sha256Hex string, size int64, err error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return "", 0, fmt.Errorf("backup: hash %s: %w", path, err)
+	}
+	defer f.Close()
+	h := sha256.New()
+	n, err := io.Copy(h, f)
+	if err != nil {
+		return "", 0, fmt.Errorf("backup: hash %s: %w", path, err)
+	}
+	return hex.EncodeToString(h.Sum(nil)), n, nil
+}