…ghands.

Most of the change is moving sets around to be static functions on
LonghandIdSet. I think I like that pattern, but I can also make the new set a
global static and add mako code to be `pub` or something.

Though I think the LonghandIdSet::foo().contains(..) pattern is nice to read :)

Differential Revision: https://phabricator.services.mozilla.com/D10653