Make a common approach for "emulated" types

Make a common approach for "emulated" types

Internal refinements to the :class:`.Enum`, :class:`.Interval`, and
:class:`.Boolean` types, which now extend a common mixin
:class:`.Emulated` that indicates a type that provides Python-side
emulation of a DB native type, switching out to the DB native type when a
supporting backend is in use.   The Postgresql :class:`.INTERVAL` type
when used directly will now include the correct type coercion rules for
SQL expressions that also take effect for :class:`.sqltypes.Interval`
(such as adding a date to an interval yields a datetime).

Change-Id: Ifb9f9d7cbd9f5990dcb2abb583193e9e92b789ad
Fixes: #4088