gnuvince Use `fs::read()` instead of `BufReader::read_to_end()`

This cuts the time needed to read large files by 2-4x.

                ~/projects/ppbert$ ppbert -2pv /tmp/large.bert2
        ppbert: read time: 0.223202367
        ppbert: parse time: 0.491945874

                ~/projects/ppbert$ ./target/release/ppbert -2pv /tmp/large.bert2
        ppbert: read time: 0.055291360
        ppbert: parse time: 0.452091441