Export `getindex!`

[matthieugomez]

Would it be possible to export getindex! defined here? It's very useful.

[tomasaschan]

I suppose we could, but it feels like a function that we maybe should have imported from base, had it existed there. It doesn't (yet), though...

@timholy, what do you say to this?

[timholy]

I've often wanted this from Base. But we can export it even if Base doesn't. The biggest argument against it is that you can do this with copy!(dest, sub(src, indexes...)), except that SubArrays don't take fractional indexes.