Override `stepper` in more collection types

[lrytz]

(source: scala/scala-java8-compat#113)

Check if there are more collections that would from a stepper override (with an existing or new stepper implementation). From the top of my head:

• mutable/immutable ArraySeq, ArrayBuffer
• mutable LinkedHashMap/LinkedHashSet (cannot use XTableStepper because that doesn't yield the elements in insertion order)
• ArrayDeque
• VectorMap

[lrytz]

To allow doing that in a minor release, it might make sense to add some overrides of stepper in 2.13.0 without actually providing a more efficient Stepper (yet). @Ichoran, what do you think?

[Ichoran]

@lrytz - LinkedHashX are linked lists when following insertion order. There's no point overriding them.

VectorMap can be overridden. The existing Vector stepper should work fine mapped, but we don't have a mapped stepper any longer.

ArraySeq and ArrayBuffer can just use the stepper for arrays.

ArrayDeque has a (slightly) more complicated indexing scheme and will need a new (slightly modified) stepper.

So I'm not sure the overrides are enough given that we need new stepper classes also. I guess we could use anonymous subclasses whose type isn't observable.

[lrytz]

We can declare new steppers private[collection], as the types don't leak out to the stepper signature.