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

Also cleaned up handling of defaults in MultiChoiceSetting.
Here's another one - feel free to ignore, I can just do it with my own accessors, and it probably just makes things unnecessarily complicated.

I currently need a generalization of `_`. For `-Yopt` there are individual flags (like `dead-code`), but also group flags, like `level:method`, which enable a subset of the individual flags. I'd like to specify that some options just enable others. `_` would be just a (customizable) default.