Represent serializable objects in a digraph, and as a result made most
object relationships implicitly defined, and use the graph to trace
references between objects to determine relationships. Edges may
also be marked as having a dependency of the object they point to,
which allows for automatic cleanup and deletion of most objects when
no longer needed.

Additionally, this allows not having to require in-memory copies of
everything when using external databases. db_sql has been rewritten
for this and now always requires a database to function. db_sql with
MySQL now requires InnoDB to make use of transactions and foreign
key constraints.