io: use a single slice for numbers

Slice takes 24 bytes of memory, while we really need only 9.
```
name                 old time/op    new time/op    delta
Transaction_Bytes-8     667ns ±17%     583ns ± 6%  -12.50%  (p=0.000 n=10+10)
GetVarSize-8            283ns ±11%     189ns ± 5%  -33.37%  (p=0.000 n=10+10)

name                 old alloc/op   new alloc/op   delta
Transaction_Bytes-8    1.01kB ± 0%    0.88kB ± 0%  -12.70%  (p=0.000 n=10+10)
GetVarSize-8             184B ± 0%       56B ± 0%  -69.57%  (p=0.000 n=10+10)

name                 old allocs/op  new allocs/op  delta
Transaction_Bytes-8      7.00 ± 0%      6.00 ± 0%  -14.29%  (p=0.000 n=10+10)
GetVarSize-8             3.00 ± 0%      2.00 ± 0%  -33.33%  (p=0.000 n=10+10)
```

Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>

pkg/io/binaryWriter.go

@@ -11,56 +11,47 @@ import (
 // from a struct with many fields.
 type BinWriter struct {
 	w   io.Writer
-	uv  []byte
-	u64 []byte
-	u32 []byte
-	u16 []byte
-	u8  []byte
 	Err error
+	uv  [9]byte
 }
 
 // NewBinWriterFromIO makes a BinWriter from io.Writer.
 func NewBinWriterFromIO(iow io.Writer) *BinWriter {
-	uv := make([]byte, 9)
-	u64 := uv[:8]
-	u32 := u64[:4]
-	u16 := u64[:2]
-	u8 := u64[:1]
-	return &BinWriter{w: iow, uv: uv, u64: u64, u32: u32, u16: u16, u8: u8}
+	return &BinWriter{w: iow}
 }
 
 // WriteU64LE writes an uint64 value into the underlying io.Writer in
 // little-endian format.
 func (w *BinWriter) WriteU64LE(u64 uint64) {
-	binary.LittleEndian.PutUint64(w.u64, u64)
-	w.WriteBytes(w.u64)
+	binary.LittleEndian.PutUint64(w.uv[:8], u64)
+	w.WriteBytes(w.uv[:8])
 }
 
 // WriteU32LE writes an uint32 value into the underlying io.Writer in
 // little-endian format.
 func (w *BinWriter) WriteU32LE(u32 uint32) {
-	binary.LittleEndian.PutUint32(w.u32, u32)
-	w.WriteBytes(w.u32)
+	binary.LittleEndian.PutUint32(w.uv[:4], u32)
+	w.WriteBytes(w.uv[:4])
 }
 
 // WriteU16LE writes an uint16 value into the underlying io.Writer in
 // little-endian format.
 func (w *BinWriter) WriteU16LE(u16 uint16) {
-	binary.LittleEndian.PutUint16(w.u16, u16)
-	w.WriteBytes(w.u16)
+	binary.LittleEndian.PutUint16(w.uv[:2], u16)
+	w.WriteBytes(w.uv[:2])
 }
 
 // WriteU16BE writes an uint16 value into the underlying io.Writer in
 // big-endian format.
 func (w *BinWriter) WriteU16BE(u16 uint16) {
-	binary.BigEndian.PutUint16(w.u16, u16)
-	w.WriteBytes(w.u16)
+	binary.BigEndian.PutUint16(w.uv[:2], u16)
+	w.WriteBytes(w.uv[:2])
 }
 
 // WriteB writes a byte into the underlying io.Writer.
 func (w *BinWriter) WriteB(u8 byte) {
-	w.u8[0] = u8
-	w.WriteBytes(w.u8)
+	w.uv[0] = u8
+	w.WriteBytes(w.uv[:1])
 }
 
 // WriteBool writes a boolean value into the underlying io.Writer encoded as
@@ -108,7 +99,7 @@ func (w *BinWriter) WriteVarUint(val uint64) {
 		return
 	}
 
-	n := PutVarUint(w.uv, val)
+	n := PutVarUint(w.uv[:], val)
 	w.WriteBytes(w.uv[:n])
 }