What one family’s schema says, as a table: its classes, the profiles it
defines and the namespaces its vocabularies bind. Generated by cimgen, or
read from RDFS at runtime (schema_source).
The one &'static copy of a field key the class tables do not hold — an
attribute the element’s class does not declare, as written in the file or
set from JSON. Each distinct key is leaked once and kept for the process;
there are as many as distinct undeclared attribute names, not fields.
An element’s fields, keyed by Class.attr. Keys are &'static: a declared
attribute’s key is its id in the class table, any other goes through
intern, so a key is allocated once per process rather than per field.