[#1525] pilorama: Use AppendUint* helpers from stdlib

gopatch:
```
@@
var slice, e expression
@@
+import "encoding/binary"

-append(slice, byte(e), byte(e >> 8))
+binary.LittleEndian.AppendUint16(slice, e)
```

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

pkg/local_object_storage/pilorama/boltdb.go

@@ -1240,7 +1240,7 @@ func (t *boltForest) fillSortedChildren(b *bbolt.Bucket, nodeIDs MultiNode, h *f
 					nodes = nil
 					length = actualLength + 1
 					count = 0
-					c.Seek(append(prefix, byte(length), byte(length>>8)))
+					c.Seek(binary.LittleEndian.AppendUint16(prefix, length))
 					c.Prev() // c.Next() will be performed by for loop
 				}
 			}
@@ -1664,7 +1664,7 @@ func internalKeyPrefix(key []byte, k string) []byte {
 	key = append(key, 'i')
 
 	l := len(k)
-	key = append(key, byte(l), byte(l>>8))
+	key = binary.LittleEndian.AppendUint16(key, uint16(l))
 	key = append(key, k...)
 	return key
 }
@@ -1679,14 +1679,10 @@ func internalKey(key []byte, k, v string, parent, node Node) []byte {
 	key = internalKeyPrefix(key, k)
 
 	l := len(v)
-	key = append(key, byte(l), byte(l>>8))
+	key = binary.LittleEndian.AppendUint16(key, uint16(l))
 	key = append(key, v...)
 
-	var raw [8]byte
+	key = binary.LittleEndian.AppendUint64(key, parent)
-	binary.LittleEndian.PutUint64(raw[:], parent)
+	key = binary.LittleEndian.AppendUint64(key, node)
-	key = append(key, raw[:]...)
-
-	binary.LittleEndian.PutUint64(raw[:], node)
-	key = append(key, raw[:]...)
 	return key
 }