Array Semigroup instance should check length

[natefaubion]

If either of the arrays are 0-length, then we should return the other as is instead of unnecessarily creating a copy.

[hdgarrood]

Do we know if JS VMs do this already? (And why not, if not?)

Even if they don't, I think doing this could have a cost for foldMap on large structures. I'd like to see benchmarks first.

[garyb]

They don't do so, as concat always shallow-copies arrays in JS by design.

Can you explain a bit more about your concern with foldMap? I'm not sure what you're thinking there.