These methods are equivalent to `AppendByte` and `AppendString`, but
their signatures are compatible with
[bytes.Buffer](https://godoc.org/bytes#Buffer) and
[bufio.Writer](https://godoc.org/bufio#Writer).

This allows to use `Buffer` where `bytes.Buffer` was expected without
extra cost of wrapping. One example is msgpack library which
expects `Writer` to implement `WriteByte` and `WriteString`, otherwise
it is wrapped which incurs extra allocation:
https://github.com/vmihailenco/msgpack/blob/master/encode.go#L63-L67