The cassandra adapter now extends Polyclay to add five new types:
sets plus four mapping variants. For Cassandra map types, keys are
must be strings because the Javascript native object data type uses
strings for fields. You can, however, map strings to strings, numbers,
booleans, and dates. The new types are named:

	set
	map:string
	map:number
	map:boolean
	map:date

Initialization isn't handled well yet (you must initialize your property before
you can use it) and there's no validation on use at all. But obj.valid()
will now correctly validate an object's data.