extended sequences don't work with the type system

[Bike]

For example, (typep extended-sequence 'sequence) => NIL and (subtypep 'my-sequence-type 'sequence) => NIL T. This is because various parts of the type system define sequence to be (or list vector) (for example there is a deftype to this effect).

See #1568 for the beginnings of a fix.

Reported by paulapatience on IRC.