add keepat! (the opposite of deleteat!)

[rfourquet]

It feels like I'm missing something, but I asked on slack and this functionality doesn't seem to exist yet in Base.
keepat!(vector, indices) deletes items at all indices of vector except those which are specified in indices.
In other words, after keepat!(a, inds), a is equal to the array which would have been obtained by a[inds].

I'm not sure about the name, it could be maybe getindex!, or the feature could be incorporated in deleteat! with a keyword.

[StefanKarpinski]

Maybe better expressed with https://github.com/mbauman/InvertedIndices.jl and deleteat!?

[rfourquet]

Maybe better expressed with https://github.com/mbauman/InvertedIndices.jl and deleteat!?

Maybe! Someone pointed out this package to me and I forgot to mention it here. But it's quite less discoverable than deleteat!, while the operation of "keeping only these indices" doesn't seem to be any less fundamental than "delete all these indices". Although it might be less often needed given that this feature isn't in Base yet and doesn't seem to have been requested before. But given how often the non-mutating equivalent vector[inds] is used, I'm quite surprised by this fact.

[vtjnash]

Sounds reasonable to me. I think the implementation can be generalized, but otherwise, lgtm conceptually. I'd merge if you finish it :)