the import structure of many core modules.
``sqlalchemy.schema`` and ``sqlalchemy.types``
remain in the top-level package, but are now just lists of names
that pull from within ``sqlalchemy.sql``.  Their implementations
are now broken out among ``sqlalchemy.sql.type_api``, ``sqlalchemy.sql.sqltypes``,
``sqlalchemy.sql.schema`` and ``sqlalchemy.sql.ddl``, the last of which was
moved from ``sqlalchemy.engine``.  ``sqlalchemy.sql.expression`` is also
a namespace now which pulls implementations mostly from ``sqlalchemy.sql.elements``,
``sqlalchemy.sql.selectable``, and ``sqlalchemy.sql.dml``.
Most of the "factory" functions
used to create SQL expression objects have been moved to classmethods
or constructors, which are exposed in ``sqlalchemy.sql.expression``
using a programmatic system.  Care has been taken such that all the
original import namespaces remain intact and there should be no impact
on any existing applications.   The rationale here was to break out these
very large modules into smaller ones, provide more manageable lists
of function names, to greatly reduce "import cycles" and clarify the
up-front importing of names, and to remove the need for redundant
functions and documentation throughout the expression package.

- 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