`Base.stack`

[mcabbott]

Now that JuliaLang/julia#43334 is merged, and available on Compat.jl, this package needs to decide what to do.

• Perhaps the lazy struct is still useful for some purposes. If the next step is broadcasting, then this can be fast.
• The eager code should just be deleted, and call Base. This was supposed to be called automatically on e.g. a stack of CuArrays, but I'm not sure that works right now.
• The name stack should not be exported. Should it be LazyStack.stack, or export lazystack?

Edit: Mostly done in #14 now. The remaining piece is:

• Base supports dims which the lazy struct does not. Perhaps it should learn, in order to have exactly the same rules?