This adds RB.FilteredCollection, which presents a filtered view of

another collection. FilteredCollection takes a dictionary of filters,

which map field names to values. For string fields, the prefix will be

compared, and for other values, direct comparison is used. This can be

expanded as needed down the road to do more complex matches.
When the main collection updates, the FilteredCollection will update as

well, according to the filter.

Unit tests pass.
Tested manually with another change that makes use of this.