Added new attribute event :meth:`.AttributeEvents.bulk_replace`.
This event is triggered when a collection is assigned to a
relationship, before the incoming collection is compared with the
existing one.  This early event allows for conversion of incoming
non-ORM objects as well.  The event is integrated with the
``@validates`` decorator.

The ``@validates`` decorator now allows the decorated method to receive
objects from a "bulk collection set" operation that have not yet
been compared to the existing collection.  This allows incoming values
to be converted to compatible ORM objects as is already allowed
from an "append" event.   Note that this means that the
``@validates`` method is called for **all** values during a collection
assignment, rather than just the ones that are new.

Change-Id: I27f59db008d9e521d31a3e30143d7cd997e4b7b3
Fixes: #3896

For a bulk replace, assume the old collection is no longer
useful to the attribute system and only send the removal events,
not actually mutated the collection.

this changes behavior significantly and also means that dispose_collection
now receives the old collection intact.

Change-Id: Ic2685c85438191f07797d9ef97833a2cfdc4fcc2
Fixes: #3913

Change-Id: I4e8c2aa8fe817bb2af8707410fa0201f938781de

the refactor in b606e47ddc54 / ticket:3457 failed
to adjust __getstate__ / __setstate__.  need to memoize
a few more things including the class itself so that we
can navigate back to "attr".

Change-Id: I4ece2a616cb8b9dac7b50763ca59e47d0f26cfdf
Fixes: #3852

Includes removal of adapt_like_to_iterable() as well
as _set_iterable(), uses __slots__ for collectionadapter,
does much less duck typing of collections.

The places inspect.getargspec was being used were causing problems for
newer Python versions.