Added dependency tracking between tables and column defaults
Don't attemp to dump temporary data used in dependency tracking

Code smell: every time we attach a random sticker on the object Pyrseas
tries to dump it to yaml. I think the approach "dump this attribute
unless we tell you not" is wrong and it should be instead "only dump
these attribute".

At least I think we may avoid dumping attributes starting with '_'.
Fixed dump of dependencies for tables/sequences

I actually wasn't dumping anything.
Dump the dependencies of tables/seqs in stable order

Another problem caused by the fact _base_map is not called, grr...
Don't dump attributes starting with '_' by default

So now there is a simple convention: is your attribute for internal use,
not to dump to yaml? Just make it _protected.
Use the base _to_map() call in table/seq too

I've had enough of fixing issue in three different places. The last one
is the random order in depends_on, which was long fixed for all the
object but still dancing around for tables.
Fixed dependency between tables and sequences

Adding the dependency of the defaults creates loops because the sequence
used to depend on the owner table but now the table defaults often
depends on the sequence. Toposort is not amused. So don't imply the
dependency of the seq on the owning table: make sure that the sequence
is always created before the table and have the OWNED BY statement
emitted by the table instead of the sequence, so it will definitely
happen after both have been created.
Fixed docstring