This fixes SI-8251: `ListBuffer.readOnly` destroys List immutability

Method `BufferLike.readOnly` now returns a view over changing state of the buffer.

`BufferLike.readOnly`, `ListBuffer.readOnly` are deprecated

Discussion:
https://groups.google.com/d/msg/scala-internals/g_-gIWgB8Os/kWazrALbLKEJ