Missing API: `Sink.write(CharSequence)`

[joffrey-bion]

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At the moment, when receiving data from a mutable CharSequence, one has to use sink.writeString(chars.toString()), which may be inefficient (copying data to create a String, just to then encode it again as UTF-8 bytes in the buffer).

It would be nice to have a Sink.write(CharSequence) (or Sink.writeUtf8(CharSequence)) extension so we can efficiently transfer characters from a CharSequence into a Buffer via UTF-8 encoding.