<array>: Should array<T, N> iterators depend on N?

Long ago, we made most of our iterators SCARY (see WG21-N2911 and WG21-N2980). This means that they don't depend on allocators, comparators, etc.

array<T, N> iterators are an interesting case, as they're still templated on N:

https://github.com/microsoft/STL/blob/6b0238d7035236af0f81917262691db701e70fe3/stl/inc/xutility#L2759-L2760

I believe that for IDL=0 it would be better to avoid depending on N. That would reduce instantiations and improve throughput. For IDL=2 I'm less certain - if we don't template on N, we have to store it at runtime (which probably isn't a big deal, IDL=2 is already costly). If we do template on N (like today), then we'd be setting up an unusual situation where iterator types observably change depending on IDL (not just their representations).

vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.

Start with stl/inc/xutility#L2759-L2760 and the linked WG21-N2911 and WG21-N2980 papers to understand the current array<T, N> iterator design. Read #169 for the vNext constraint; done means deciding how IDL=0 and IDL=2 should depend on N and implementing the resulting ABI-breaking change once vNext is available.