type stability and alloc reduction

[rafaqz]

This PR:

• fixes type instability due to the possibility of zero length iterators
• uses sort! instead of sortperm in leafnodes/parentnodes as the vector is not used after the sort anyway
• reduces memory in a few places, including by reusing scratch in sort!

[asinghvi17]

@maxfreu sorry, I seem to have missed your comment here.
#7 (comment)

We're using this quite heavily in GeometryOps.jl now (see JuliaGeo/GeometryOps.jl#259) so would be happy to either co-maintain or move the repo to JuliaGeo, depending on your preference.

[rafaqz]

One more optimisation available is not materialising the extents and indices in leaf nodes, and just keeping the view over extent/index tuple that Iterators.partition gives us. This would make indexing fractionally slower but would remove a lot of allocations.

I tried this and somehow it wasn't faster

[maxfreu]

Thanks guys! Pretty cool to receive a PR from you :) If you have no further tweaks, I'll merge.

[rafaqz]

No more changes on my side!

But you can expect more PRs, we will be using this everywhere :)

[maxfreu]

love it, thanks! <3

btw: I don't remember why I used sortperm in the first place xD