- refactor of declarative, break up into indiviudal methods

- refactor of declarative, break up into indiviudal methods
that are now affixed to _MapperConfig
- declarative now creates column copies ahead of time
so that they are ready to go for a declared_attr
- overhaul of declared_attr; memoization, cascading modifier
- A relationship set up with :class:`.declared_attr` on
a :class:`.AbstractConcreteBase` base class will now be configured
on the abstract base mapping automatically, in addition to being
set up on descendant concrete classes as usual.
fixes #2670
- The :class:`.declared_attr` construct has newly improved
behaviors and features in conjunction with declarative.  The
decorated function will now have access to the final column
copies present on the local mixin when invoked, and will also
be invoked exactly once for each mapped class, the returned result
being memoized.   A new modifier :attr:`.declared_attr.cascading`
is added as well. fixes #3150
- the original plan for #3150 has been scaled back; by copying
mixin columns up front and memoizing, we don't actually need
the "map properties later" thing.
- full docs + migration notes