Delete IndexedSeq instances and operations

We know the complexity problems of Vector but it's not possible to talk about the computational complexity of IndexedSeq because it is abstract.

I don't think the abstraction is at all useful and it prevents us from ensuring that we're making performance improvements (e.g. improving semigroup append for Vector may actually take another IndexedSeq backwards, we just don't know).

I'm happy to close this PR if IndexedSeq does have some value which I'm not aware of.