Introduce CASNodeSchema, which is the common base class for
building a schema that can be used to represent an object
inside CAS. The schema describes the ways to encode and decode
between CAS objects and their representative objects.
NamedValuesSchema is a basic schema that describes a sorted
list of named nodes, where each node is an object. The names are
stored in the root object, allowing users to decide whether to
load the objects based on their names.
