into core expression.   we need to make a sql.elements.Slice object,
which isn't a SQL standard thing; however there's no other place to
put this as long as we want slice handling to have some meaning
in Core.  Basically any other backend that ever wants to use the []
operator, I don't want to re-implement everything for PG on that
backend as well.   this will need to have backend-agnostic tests
in tests/sql/test_operator.py.

get PG stuff working

size of the many per-column objects we're hitting, but somehow the overall memory is
hardly being reduced at all in initial testing

return type is not strictly assumed to be boolean; it now
returns a :class:`.Boolean` subclass called :class:`.MatchType`.
The type will still produce boolean behavior when used in Python
expressions, however the dialect can override its behavior at
result time.  In the case of MySQL, while the MATCH operator
is typically used in a boolean context within an expression,
if one actually queries for the value of a match expression, a
floating point value is returned; this value is not compatible
with SQLAlchemy's C-based boolean processor, so MySQL's result-set
behavior now follows that of the :class:`.Float` type.
A new operator object ``notmatch_op`` is also added to better allow
dialects to define the negation of a match operation.
fixes #3263

sqlalchemy/orm, sqlalchemy/event, sqlalchemy/testing

to get all flake8 passing

implements PG's to_tsquery('regconfig', 'arg') pattern. fixes #3078

:meth:`.Operators.__or__` and :meth:`.Operators.__invert__`
operator overload methods could not be overridden within a custom
:class:`.TypeEngine.Comparator` implementation.
fixes #3012

renders "BETWEEN SYMMETRIC".  Also added a new negation operator
"notbetween_op", which now allows an expression like ``~col.between(x, y)``
to render as "col NOT BETWEEN x AND y", rather than a parentheiszed NOT
string.  fixes #2990