bufio: consider adding an option to provide a buffer to bufio.Scanner #11702
Description
The fixed size of the internal buffer inside the Scanner type bothered some people when the API was proposed, but it was decided not to complicate the interface by providing a configuration.
The new io.CopyBuffer routine suggests a different model: Allow the user to provide a buffer, which not only gives control but also permits the user to manage/reuse memory when that is appropriate.
Maybe do this for 1.6.