- The type/expression system now does a more complete job

- The type/expression system now does a more complete job
of determining the return type from an expression
as well as the adaptation of the Python operator into
a SQL operator, based on the full left/right/operator
of the given expression.  In particular
the date/time/interval system created for Postgresql
EXTRACT in [ticket:1647] has now been generalized into
the type system.   The previous behavior which often
occured of an expression "column + literal" forcing
the type of "literal" to be the same as that of "column"
will now usually not occur - the type of
"literal" is first derived from the Python type of the
literal, assuming standard native Python types + date
types, before falling back to that of the known type
on the other side of the expression.  Also part
of [ticket:1683].