ZipRange facility

[acully-vmware]

I haven't done the work to make this merge-able, but it may be of interest.

[jeffparsons]

Thanks for this, @acully-vmware! I will have a read and think about the best way to expose this sort of thing generically in RangeMap.

Aside: More than ever I'm longing for a cursor API for BTreeMap in std, because it would make this sort of thing (and the rest of rangemap) so much simpler. I had a go at implementing one a couple of years back, but a combination of really surprising things in the BTreeMap implementation and issues with the build system for rustc and friends (causing extremely slow cycle time) led me to giving up after a while. My understanding is that both of those issues have improved significantly since I last looked, so maybe I should have another go at that.

For the next little while, though, whatever time I can spare to look at RangeMap I think I should spend beefing up the test suite, to make sure there aren't any other bugs like #38 lurking in there.

[jeffparsons]

@acully-vmware Thanks again for this. I'm doing a bit of tidying up so I'm going to close this PR for now. I may add some of this functionality later (at which point I can look back at this PR for inspiration) but I'll wait until BTreeMap cursors are stabilized.