introduce implementation for ConstSortedMap
polishing, javadoc, and unit tests for BasicConstList
polishing, javadoc, and unit tests for BasicConstSet and BasicConstSortedSet
polishing, javadoc, and unit tests for BasicConstMap
polishing, javadoc, and unit tests for BasicConstSortedMap
check type of first element/key added to a ConstSortedSet/Map, otherwise the collection will get corrupted.