- Fixed bug in TypeDecorator whereby the dialect-specific

- Fixed bug in TypeDecorator whereby the dialect-specific
type was getting pulled in to generate the DDL for a
given type, which didn't always return the correct result.

- TypeDecorator can now have a fully constructed type
specified as its "impl", in addition to a type class.

- TypeDecorator will now place itself as the resulting
type for a binary expression where the type coercion
rules would normally return its impl type - previously,
a copy of the impl type would be returned which would
have the TypeDecorator embedded into it as the "dialect"
impl, this was probably an unintentional way of achieving
the desired effect.

- TypeDecorator.load_dialect_impl() returns "self.impl" by
default, i.e. not the dialect implementation type of
"self.impl".   This to support compilation correctly.
Behavior can be user-overridden in exactly the same way
as before to the same effect.