pr: don't convert to String when storing lines to print

[jfinkels]

In pr, the bytes are read from the input file as bytes, converted to String for storage, then converted back to bytes for writing to stdout. Instead, we should just buffer the bytes directly without going through String.

Originally posted by @sylvestre in #10303 (comment)

[venoosoo]

Hi, I'd like to work on this.

[cakebaker]

@venoosoo sure, go ahead :)