Adds a new variant to the "isnew" state within entity loading
for isnew=False, but the load path is new.  This is to address
the use case of an entity appearing in multiple places in
the row in a more generalized way than the fixes in [ticket:3431],
[ticket:3811] in that loading.py will be able to tell the
populator that this row is not "isnew" but is a "new" path
for the entity.   For the moment, the new information is only
being applied to the use of "quick" populators so that
simple column loads can take place on top of a deferred loader
from elsewhere in the row.

As part of this change, state.load_path() will now always
be populated with the "path" that was in effect when this state
was originally loaded, which for multi-path loads of the
same entity is still non-deterministic.  Ideally there'd be some
kind of "here's all the paths that loaded this state and how"
type of data structure though it's not clear if that could be
done while maintaining performance.

Fixes: #3822
Change-Id: Ib915365353dfcca09e15c24001a8581113b97d5e

Discovered during testing for [ticket:3822], the rule added
for [ticket:1495] will fail if the source object has propagated
options set up, which add elements to query._current_path.

Fixes: #3824
Change-Id: I3d96c96fee5f9b247f739d2136d18681ac61f2fe

The second 'if' condition was never called because the original
condition always returns.

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

Change-Id: I468fe70168804b08cf0d5e8b57e235bca904b1ff

Also improves some naming in the examples.

Change-Id: I51e5b1d9a730885aed10e5e6ade2123f5e736359
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/306

Change-Id: I5ad44362515908592f1e8b1e6254a5270d43234a
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/295