Feature request: Allow syntax @SVector 2:N-1

[DNF2]

The @SVector macro currently supports functions like rand, fill and so on. Is it possible to support StepRanges? For example, could

ind = @SVector 2:2:N-1

where N is a type parameter, be allowed?

One use case would be convenient slicing of SArrays, as discussed here, without the need for writing a custom function for each different slice.

This request might be a special case of the comprehension syntax, which cannot currently make use of type parameters.

[andyferris]

Yes, I think some way of doing this should be provided.

Unless we can find an easy shortcut, the roadmap in my head is:

• Add a static unit range type, which is returned from e.g. indices(::SVector). This would be a dataless "singleton"
• Maybe add another static step range type. With a pure constructor we can have SStepRange(2,2,N-1) deal with type parameters at run-time. The biggest problem here is actually the N-1...
• Define conversions from these to SVector
• Make the macro do this / define a new @SA macro that does multiple things.

For now, I would define an extra pure function this way:

Base.@pure function myrange(N::Int)
    r = 2:2:N-1
    SVector((r...))
end

a[myrange(N)]

[c42f]

I think this is basically a dup of #97 / #26