[#19] transformer: Do not allocate intermeate slice for hashers

```
name                 old time/op    new time/op    delta
Transformer/small-8    73.7µs ±15%    72.4µs ±16%    ~     (p=0.604 n=10+9)
Transformer/big-8       1.36s ± 4%     1.36s ± 8%    ~     (p=0.579 n=10+10)

name                 old alloc/op   new alloc/op   delta
Transformer/small-8    7.67kB ± 0%    7.57kB ± 0%  -1.36%  (p=0.000 n=10+10)
Transformer/big-8      49.0kB ± 0%    48.3kB ± 0%  -1.48%  (p=0.000 n=10+10)

name                 old allocs/op  new allocs/op  delta
Transformer/small-8       101 ± 0%        98 ± 0%  -2.97%  (p=0.000 n=10+10)
Transformer/big-8         609 ± 0%       591 ± 1%  -3.00%  (p=0.000 n=10+9)
```

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

object/transformer/transformer.go

@@ -4,7 +4,6 @@ import (
 	"crypto/ecdsa"
 	"crypto/sha256"
 	"fmt"
-	"io"
 
 	"github.com/TrueCloudLab/frostfs-sdk-go/checksum"
 	"github.com/TrueCloudLab/frostfs-sdk-go/object"
@@ -25,8 +24,6 @@ type payloadSizeLimiter struct {
 
 	previous []oid.ID
 
-	chunkWriter io.Writer
-
 	splitID *object.SplitID
 
 	parAttrs []object.Attribute
@@ -118,21 +115,6 @@ func (s *payloadSizeLimiter) initializeCurrent() {
 	// create payload hashers
 	s.writtenCurrent = 0
 	s.initPayloadHashers()
-
-	// compose multi-writer from target and all payload hashers
-	ws := make([]io.Writer, 0, 1+len(s.currentHashers)+len(s.parentHashers))
-
-	ws = append(ws, s.NextTarget)
-
-	for i := range s.currentHashers {
-		ws = append(ws, s.currentHashers[i].hasher)
-	}
-
-	for i := range s.parentHashers {
-		ws = append(ws, s.parentHashers[i].hasher)
-	}
-
-	s.chunkWriter = io.MultiWriter(ws...)
 }
 
 func (s *payloadSizeLimiter) initPayloadHashers() {
@@ -269,7 +251,7 @@ func (s *payloadSizeLimiter) writeChunk(chunk []byte) error {
 			cut = leftToEdge
 		}
 
-		if _, err := s.chunkWriter.Write(chunk[:cut]); err != nil {
+		if err := s.writeHashes(chunk[:cut]); err != nil {
 			return fmt.Errorf("could not write chunk to target: %w", err)
 		}
 
@@ -285,6 +267,24 @@ func (s *payloadSizeLimiter) writeChunk(chunk []byte) error {
 	}
 }
 
+func (s *payloadSizeLimiter) writeHashes(chunk []byte) error {
+	_, err := s.NextTarget.Write(chunk)
+	if err != nil {
+		return err
+	}
+
+	// The `Write` method of `hash.Hash` never returns an error.
+	for i := range s.currentHashers {
+		_, _ = s.currentHashers[i].hasher.Write(chunk)
+	}
+
+	for i := range s.parentHashers {
+		_, _ = s.parentHashers[i].hasher.Write(chunk)
+	}
+
+	return nil
+}
+
 func (s *payloadSizeLimiter) prepareFirstChild() {
 	// initialize split header with split ID on first object in chain
 	s.current.InitRelations()