Revisit `toString` content

[whyoleg]

While reviewing #174 I found that current toString functions are not adapted after Okio -> kotlinx-io migration:

• should be buffered(this) or this.buffered()
  

  kotlinx-io/core/common/src/RealSink.kt

  Line 151 in 03845ea

  override fun toString() = "buffer($sink)"

• should be asOutputStream()
  

  kotlinx-io/core/jvm/src/SinksJvm.kt

  Line 85 in 03845ea

  override fun toString() = "${this@asOutputStream}.outputStream()"

• same for sources (may be somewhere else)