Fix API NewWriter api to receive io.Writer and not a filename

[Daker Fernandes Pinheiro]

Making this, conforms the API as the other examples in the standard library.

[Kerby Shedden]

Thanks for the comment. I agree the naming is unconventional but changing it would involve a compatibility break (the requested functionality is already present under names ReaderFromStream and WriterFromStream).

At this point, I added more conventional alternative names NewFileWriter and NewFileReader and marked NewReader and NewWriter for possible incompatible changes in the future.

[Kerby Shedden]

I updated the API so that the names are more conventional, as suggested above.