Consider std::io::Stdin instead of File('/dev/stdin')

[bovee]

I'm not sure if there's a performance difference (in which case please feel free to add a comment and disregard this), but I think opening /dev/stdin instead of using std::io::Stdin might limit the platforms fasten works on to only POSIX ones (i.e. probably not Windows). Not a high priority, but an easy search/change and probably a good beginner bug.

[lskatz]

Thank you for that tip! I am going to leave this ticket open for when I start refactoring.