the vast majority of file lines are spent on documentation, which moves
into package __init__.  The core declarative idea lives in base and
is back down to its originally low size of under 500 lines.  The various
helpers and such move into api.py, and the full span of string lookup
moves into a new module clsregistry.  the rest of declarative only
refers to two functions in clsregistry in three places inside of base.
- [feature] Declarative now maintains a registry
of classes by string name as well as by full
module-qualified name.   Multiple classes with the
same name can now be looked up based on a module-qualified
string within relationship().   Simple class name
lookups where more than one class shares the same
name now raises an informative error message.
[ticket:2338]
- lots of tests to ensure the new weak referencing memory management
is maintained by the new class registry system.   this ticket was
served very well by waiting to do #2526 first, else this would
have needed to be rewritten anyway.

- modify inspection system:
	1. raise a new exception for any case where the inspection
        context can't be returned.  this supersedes the "not mapped"
        errors.
        2. don't configure mappers on a mapper inspection.  this allows
        the inspectors to be used during mapper config time.  instead,
        the mapper configures on "with_polymorphic_selectable" now,
        which is needed for all queries
- add a bunch of new "is_XYZ" attributes to inspectors
- finish making the name change of "compile" -> "configure", for some reason
this was only done partially

    in declarative_base() is now a
    WeakValueDictionary.  So subclasses of
    "Base" that are dereferenced will be
    garbage collected, *if they are not
    referred to by any other mappers/superclass
    mappers*.  [ticket:2526]

such as ``.. versionadded::``, ``.. versionchanged::`` and ``.. deprecated::``.

whereby the precedence of columns
in a joined-table, composite
column (typically for id) would fail to
be correct if the columns contained
names distinct from their attribute
names.  This would cause things like
primaryjoin conditions made against the
entity attributes to be incorrect.  Related
to [ticket:1892] as this was supposed
to be part of that, this is [ticket:2491].
Also in 0.7.8.

how to make a public API for this anyway

called after the __prepare__() step, if any, so that everything to
do with the mapping occurs after the table is reflected.