This patch also begins to formally describe the entity_id/entity_table
references, introducing a new first-class object CRM_Core_EntityReference.
We still need some way to describe the restrictions on the possible values of
entity_table for each DAO, so that findReferences() doesn't have to scan
all tables using a generic pointer.

Note that I've broken compatibility with PEAR links(), this would be
easy to reimplement if we need, but it was not being unused in core.

This is an improvement on CRM/Core/DAO/listAll.php, wrapping in a class
and making the table names available as well.

This is an improvement on CRM/Core/DAO/listAll.php, wrapping in a class
and making the table names available as well.