Add FileUtils.compare_file (and aliases cmp and identical?) + Fix behavior or File.readlines

1- Add FileUtils.compare and its aliases
2- Fix File.readlines behavior:
Before the fix, the trailing \n wer removed from the output, now they are kept like when not using fakefs.
WARNING, this will cause some test to fail if they do a strict comparison, but behavior is now aligned with ruby beahvior.