Several corrections as per Daniel and Baron's feedback.

Still TODO: Attributes still have a single method that doubles
as a getter and setter. The constructor for Diskstats is still
weird -- A tad more stricter than it should be, if anything.
->print_rest is still rest, even though that's hardly
memorable, mostly because of a lack of ideas on what to
rename it. The main loop in the Menu is still a while (1).

As a nice perk, it's nearly twice as fast now! It also adds a
_very_ experimental --memory-for-speed argument, which
turns on memoization for the current biggest bottleneck.

t/pt-diskstats/pt-diskstats.t

@@ -43,7 +43,7 @@ for my $ext ( qw( all disk sample ) ) {
          tie local *STDIN, "Test";
          my $file = File::Spec->catfile( $trunk, "t", "pt-diskstats",
                                              "samples", $filename );
-         pt_diskstats->main(
+         pt_diskstats::main(
                   "--group-by" => $ext,
                   "--columns"  => "cnc|rt|mb|busy|prg",
                   "--zero-rows",