Add SOneTo as AbstractUnitRange to represent axes(::StaticArray)

[andyferris]

This has been on the to-do list for 2 years! 🎉

Now axes(::StaticArray) will return a tuple of "static unit ranges / slices". The SOneTo type lives under AbstractUnitRange so it can work properly with Base, and is therefore not a StaticArray subtype.

One interesting side-effect is what happens to collect:

julia> v = SVector(1,2,3)
3-element SArray{Tuple{3},Int64,1,3}:
 1
 2
 3

julia> [x for x in v]
3-element SizedArray{Tuple{3},Int64,1,1}:
 1
 2
 3

This matches the OffsetArrays package... basically if you want to mess with axes your behavior is automatically used by collect, and in particular for offsets (Slices) you kind of need to rely on this to work out properly. I guess we should roll with this for now. If we can get generator/comprehension syntax working fast (constructing SVectors with optimal code) that could be pretty neat.

I believe the benefits of this will eventually be far-reaching. In particular the axes and keys of an array can now be directly used as an indication of it's static-sizedness. This is more-or-less a trait that is already plumbed into many systems (like collect). Hopefully this will make dealing with "wrapped up" types like Diagonal with SVector more natural.

Closes #476

[andyferris]

Ready or not, here it comes...