`WriteArray` involves reflection, it makes sense to optimize
serialization of transactions and application logs which are serialized
constantly. Adding case in a type switch in `WriteArray` is not an
option because we don't want new dependencies for `io` package.
```
name old time/op new time/op delta
AppExecResult_EncodeBinary-8 852ns ± 3% 656ns ± 2% -22.94% (p=0.000 n=10+9)
name old alloc/op new alloc/op delta
AppExecResult_EncodeBinary-8 448B ± 0% 376B ± 0% -16.07% (p=0.000 n=10+10)
name old allocs/op new allocs/op delta
AppExecResult_EncodeBinary-8 7.00 ± 0% 5.00 ± 0% -28.57% (p=0.000 n=10+10)
```
```
name old time/op new time/op delta
Transaction_Bytes-8 1.29µs ± 3% 0.76µs ± 5% -41.52% (p=0.000 n=9+10)
name old alloc/op new alloc/op delta
Transaction_Bytes-8 1.21kB ± 0% 1.01kB ± 0% -16.56% (p=0.000 n=10+10)
name old allocs/op new allocs/op delta
Transaction_Bytes-8 12.0 ± 0% 7.0 ± 0% -41.67% (p=0.000 n=10+10)
```
Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>
291a29af1e