Make Source to/from InputStream and Sink to/from OutputStream consistent

[vdshb]

On JVM target there is a strange (at the first glance) decision to make
Source.asInputStream(): InputStream with InputStream.asSource(): RawSource
and
Sink.asOutputStream(): OutputStream with OutputStream.asSink(): RawSink

Hence such simple and intuitive pipelines:

import kotlinx.io.Buffer
import kotlinx.io.Source
import kotlinx.io.asInputStream
import kotlinx.io.asOutputStream
import kotlinx.io.asSink
import kotlinx.io.asSource

fun main() {
  Buffer().asInputStream().asSource().asInputStream()
  Buffer().asOutputStream().asSink().asOutputStream()
}

are impossible (broken at the last chain link) with current API.

(Of cause I don't need to make such chains, but I care about API consistency and about ability to convert in/from InputStream easily)

I think it might be good to consider:

option 1)
refactor Source.asInputStream() -> RawSource.asInputStream()
refactor Sink.asOutputStream() -> RawSink.asOutputStream()

oprion 2)
rename InputStream.asSource(): RawSource -> InputStream.asRawSource(): RawSource
rename OutputStream.asSink(): RawSink -> OutputStream.asRawSink(): RawSink
add OutputStream.asSink(): Sink
add InputStream.asSource(): Source

[Chuckame]

The option 2 seems to be indirectly linked to #362 as the name RealSource should be named BufferedSource, same for sink, so it would be weird to directly go to a buffered source/sink from a jdk stream