Generalize getindex for AbstractUnitRanges

[timholy]

It turns out we were returning a Vector{Int} for operations that should return a Range

[tkelman]

@nanosoldier runbenchmarks(ALL, vs = ":master")

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here. cc @jrevels

[timholy]

I've tested those regressions; 2/3 are clearly noise (and in fact go the opposite way in my hands). The simd benchmark appeared to have a 12% regression when I tested, but looking at the code I don't think this is related.