Drop 'compatibility' with reference implementation's serialization

The reference implementation stores values we don't use and doesn't
have a documented spec. It uses two different encodings, but we only
need one. It also uses float weights, while we use integers.

Compatibility was a bit of a sham before anyway, so it's best to drop
the charade and use our own scheme.

serde.go

@@ -7,21 +7,16 @@ import (
 	"io"
 )
 
-const encodingVersion = int32(1)
+const (
+	magic           = int16(0xc80)
+	encodingVersion = int32(1)
+)
 
 func marshalBinary(d *TDigest) ([]byte, error) {
 	buf := bytes.NewBuffer(nil)
 	w := &binaryBufferWriter{buf: buf}
+	w.writeValue(magic)
 	w.writeValue(encodingVersion)
-
-	var min, max float64
-	if len(d.centroids) > 0 {
-		min = d.centroids[0].mean
-		max = d.centroids[len(d.centroids)-1].mean
-	}
-
-	w.writeValue(min)
-	w.writeValue(max)
 	w.writeValue(d.compression)
 	w.writeValue(int32(len(d.centroids)))
 	for _, c := range d.centroids {
@@ -37,17 +32,19 @@ func marshalBinary(d *TDigest) ([]byte, error) {
 
 func unmarshalBinary(d *TDigest, p []byte) error {
 	var (
-		ev       int32
-		min, max float64
-		n        int32
+		mv int16
+		ev int32
+		n  int32
 	)
 	r := &binaryReader{r: bytes.NewReader(p)}
+	r.readValue(&mv)
+	if mv != magic {
+		return fmt.Errorf("invalid header magic value, data might be corrupted: %d", mv)
+	}
 	r.readValue(&ev)
 	if ev != encodingVersion {
 		return fmt.Errorf("invalid encoding version: %d", ev)
 	}
-	r.readValue(&min)
-	r.readValue(&max)
 	r.readValue(&d.compression)
 	r.readValue(&n)
 	if r.err != nil {