ORM attributes can now be assigned any object that is has a
``__clause_element__()`` attribute, which will result in inline
SQL the way any :class:`.ClauseElement` class does.  This covers other
mapped attributes not otherwise transformed by further expression
constructs.

As part of this, it was considered that we could add
__clause_element__() to ClauseElement, however this causes endless loops
in a "while" pattern and this pattern has been identified in third
party libraries.  Add a test to ensure we never make that change.

Change-Id: I9e15b3f1c4883fd3909acbf7dc81d034c6e3ce1d
Fixes: #3802

The "raise_on_sql" option differentiates from "raise" in that
firing a lazy loader is OK as long as it does a simple
get from identity map.   Whereas "raise" is more useful
for the case that objects are to be detached.

As part of this, refactors the strategy initiation logic
a bit so that a LoaderStrategy itself knows what "key" was used
to create it, thus allowing variants of a single strategy
based on what the "lazy" argument is.  To achieve this we
have to also get rid of _get_strategy_by_cls().

Everything here is internal with the one exception of an apparently
undocumented, but not underscored, "strategy_class" key
on relationship().   Though it's not clear what
"strategy_class" accomplishes; at this point the strategy
system is extensible using Property.strategy_for().

Fixes: #3812
Change-Id: I812ad878ea5cf764e15f6f71cb39eee78a645d88

The change in #3431 still checks that the instance() is
non-None, deferring to other loading schemes if it is.
These columns are dedicated towards the entity however, so if the value
is None, we should set it.  If it conflicts, we are detecting that
in any case.

Change-Id: I223768e2898e843f953e910da1f9564b137d95e4
Fixes: #3811

Add checks in spots where state.obj() might be late-GC'ed before
we get a chance to call the event.   There may be more cases
of these which we should address as they come up.  The Session
should always be maintaining strong refs to objects that have
pending operations left on them, so for these cases we need
to ensure that ref remains long enough for the event to be called.

Change-Id: I1a7c7bc57130acc11f54ad55924af2e36ac75101
Fixes: #3808

and nested attributes and what these mean
- improve linking for after_transaction_create() / after_transaction_end()
events
- add public .parent attribute to detect top-level transaction within
these events

Change-Id: Ie7382bc8fe5de226160dcb6a5019e19fcc5af38e

Fixed bug where joined eager loading would fail for a polymorphically-
loaded mapper, where the polymorphic_on was set to an un-mapped
expression such as a CASE expression.

Change-Id: Iffe68196aaac592165c89684f09f4c06cd78ce54
Fixes: #3800