add reduce(vcat) overload

[aplavin]

Without it, reduce(vcat) seems to fall back to the general reduce with much worse performance.

[piever]

I would have thought this should have worked without overloading anything, but I suspect the problem is a missing similar method. Do you get a fast reduce(vcat, structarrays) if you check out #94 ?

[aplavin]

Actually, I got somewhat confused: the issue isn't performance, it's that currently reduce(vcat) returns a vector of namedtuples when given a vector of structarrays. And this does get fixed by #94 - a structarray is returned! Any plans to merge that?

[piever]

That got a bit stale, I've rebased and cleaned up in #218. Other than a confusing ambiguity (when OffsetArrays are involved), it should be possible to merge it quickly. Feel free to comment there if you have ideas on the OffsetArrays issue.

[aplavin]

Thanks for that update! With #218, reduce(vcat, structarrays) properly returns a structarray, and is quite performant - comparable to reduce(vcat, vectors_of_namedtuples), even though somewhat slower.