Add NSSet/NSMutableSet filters. Add queue support.

* All map/select/reject variants now optionally take a queue. A nil
  queue will cause it to run without using one, obviously.
* Reduce does not use a queue because I'm not aware of any good way to
  do this asynchronously aside from intelligently breaking stuff down
  into smaller collections and reducing those in turn.
* Use of queues more or less requires manual retain/release - this is
  done by using CFRetain/Release as needed.
* Most of the code inside methods has been reduced to calls into static
  functions that are a little more flexible than previous.
Rename NSArray+Filters to NSFilters.

The filename didn't really make sense anymore. Also adopt the SNOW_
prefix for the header guard.
Fix test for existing character in bitmap font.

Previously did absolutely nothing other than waste cycles/memory - which
would have been unnoticeable really since the chance of it actually
resulting in a duplicate character is low.

Now creates a set of existing characters and checks if the character is
contained by the set.