Consider alternative Sink.writeString implementations on JVM

[fzhinkin]

On JVM, instead of reading each character separately and then encoding it to UTF-8 and writing to a buffer, it might be faster to:

• extract chars to a CharArray and then iterate over it;
• simply use toByteArray.

For other libraries, namely kotlinx.serialization, some of these approaches performed better. While quick ad-hoc experiments didn't show any pros for kotlinx-io, it does make sense to investigate it thoroughly.