Consider changing filesystem default methods for Sink/Source

[qwwdfsad]

Currently, the file system interface provides the following methods:

fun sink(...): RawSink
fun source(): RawSource

yet for user-specific needs, one most likely needs Sink and Source interfaces that we encourage over Raw counterparts.

It would be nice to have something like:

// Part of the core interface

fun sink(...): Sink = rawSink().buffered()
fun rawSink(...): RawSink  = ...

and the corresponding source counterpart