pep-484: ORM public API, constructors

pep-484: ORM public API, constructors

for the moment, abandoning using @overload with
relationship() and mapped_column().  The overloads
are very difficult to get working at all, and
the overloads that were there all wouldn't pass on
mypy.  various techniques of getting them to
"work", meaning having right hand side dictate
what's legal on the left, have mixed success
and wont give consistent results; additionally,
it's legal to have Optional / non-optional
independent of nullable in any case for columns.
relationship cases are less ambiguous but mypy
was not going along with things.

we have a comprehensive system of allowing
left side annotations to drive the right side,
in the absense of explicit settings on the right.
so type-centric SQLAlchemy will be left-side
driven just like dataclasses, and the various flags
and switches on the right side will just not be
needed very much.

in other matters, one surprise, forgot to remove string support
from orm.join(A, B, "somename") or do deprecations
for it in 1.4.   This is a really not-directly-used
structure barely
mentioned in the docs for many years, the example
shows a relationship being used, not a string, so
we will just change it to raise the usual error here.

Change-Id: Iefbbb8d34548b538023890ab8b7c9a5d9496ec6e