io: simplify WriteBytes()

Which speeds it up at least twofold for a typical 32-bytes write (and that's
for a very naïve test that allocates new BufBinWriter on every iteration):

pkg: github.com/CityOfZion/neo-go/pkg/io
BenchmarkWriteBytes-8           10000000               124 ns/op
BenchmarkWriteBytesOld-8         5000000               251 ns/op

pkg/io/binaryWriter.go

@@ -95,13 +95,16 @@ func (w *BinWriter) WriteVarUint(val uint64) {
 
 // WriteBytes writes a variable byte into the underlying io.Writer without prefix.
 func (w *BinWriter) WriteBytes(b []byte) {
-	w.WriteLE(b)
+	if w.Err != nil {
+		return
+	}
+	_, w.Err = w.w.Write(b)
 }
 
 // WriteVarBytes writes a variable length byte array into the underlying io.Writer.
 func (w *BinWriter) WriteVarBytes(b []byte) {
 	w.WriteVarUint(uint64(len(b)))
-	w.WriteLE(b)
+	w.WriteBytes(b)
 }
 
 // WriteString writes a variable length string into the underlying io.Writer.

pkg/io/binaryrw_test.go

@@ -210,6 +210,11 @@ func TestWriteBytes(t *testing.T) {
 	buf := bw.Bytes()
 	assert.Equal(t, 4, len(buf))
 	assert.Equal(t, byte(0xde), buf[0])
+
+	bw = NewBufBinWriter()
+	bw.Err = errors.New("smth bad")
+	bw.WriteBytes(bin)
+	assert.Equal(t, 0, bw.Len())
 }
 
 type testSerializable uint16