Move $size to the new model
Move $exists to the new model

Fixes two incompatibilities:
- {$exists: false} did the wrong logic when there were multiple branches
   (we (poorly) special-cased $not and $nin but not this negative case)
- No longer require the argument to $exists to be a boolean;
  {$exists: 0} and {$exists: 1} should work, eg
Move $type to the new model

Requires another special option, yay.