Convert MonoClassExt to class properties (#4022)

Convert MonoClassExt to class properties (#4022)

* [runtime] Store the class failure data as a property.

* [runtime] Store the list of nested classes as a class property.

* [runtime] Move the property relation information from MonoClassExt into a class property.

* [runtime] Move the event relation information from MonoClassExt into a class property.

* [runtime] Move the field_def_values field from MonoClassExt into a class property.

* [runtime] Move the declsec_flags field from MonoClassExt into a class property.

* [runtime] Remove unused MonoClassExt infrastructure.

* [runtime] Add some API docs.