io: add type-specific read/write methods

This seriously improves the serialization/deserialization performance for
several reasons:
 * no time spent in `binary` reflection
 * no memory allocations being made on every read/write
 * uses fast ReadBytes everywhere it's appropriate

It also makes Fixed8 Serializable just for convenience.

pkg/core/blockchain.go

@@ -1451,7 +1451,7 @@ func (bc *Blockchain) verifyBlockWitnesses(block *Block, prevHeader *Header) err
 func hashAndIndexToBytes(h util.Uint256, index uint32) []byte {
 	buf := io.NewBufBinWriter()
 	buf.WriteBytes(h.BytesLE())
-	buf.WriteLE(index)
+	buf.WriteU32LE(index)
 	return buf.Bytes()
 }