- fix and test weakref cleanout for WeakSequence, [ticket:2794]

by the ORM to iterate mapper hierarchies; under the Jython interpreter
this implementation wasn't ordered, even though cPython and Pypy
maintained ordering.  Also in 0.8.3.
[ticket:2794]

Conflicts:
	doc/build/changelog/changelog_09.rst
	lib/sqlalchemy/util/_collections.py

labeled columns when apply_labels() is used; this mode
produces a SELECT where each column is labeled as in
<tablename>_<columnname>, to remove column name collisions
for a multiple table select.   The fix is that if two labels
collide when combined with the table name, i.e.
"foo.bar_id" and "foo_bar.id", anonymous aliasing will be
applied to one of the dupes.  This allows the ORM to handle
both columns independently; previously, 0.7
would in some cases silently emit a second SELECT for the
column that was "duped", and in 0.8 an ambiguous column error
would be emitted.   The "keys" applied to the .c. collection
of the select() will also be deduped, so that the "column
being replaced" warning will no longer emit for any select()
that specifies use_labels, though the dupe key will be given
an anonymous label which isn't generally user-friendly.
[ticket:2702]

- other cleanup
- don't need compat.decimal, that approach never panned out. hopefully
outside libs aren't pulling it in, they shouldn't be