-Ystatistics accepts a list of phases for which to print stats

Also cleaned up handling of defaults in MultiChoiceSetting.
One could look at the API the other way around, that `value` is just "what the user supplied" and `contains` is the test method for if something is enabled. `PhasesSetting.contains` is the (maybe bad) precedent.  It would be nice if `PhasesSetting` used `_` and also that a `PhasesSetting` could be mixed-in with `Multichoice`, but that is out of scope.

I'll think about it during review, but it would be nice if the is-enabled test is fast.  And, whoops, I just went to check that lint warnings are just a flag check, and I see they've been upgraded to a look-up.  At that point, one inclines to an enum set.