Support efficient rseq on int maps
As discussed previously in https://github.com/ztellman/immutable-int-map/pull/1
I note that the existing Seq implementation is not lazy, so i've just implemented a basic reverse iterator around this rather than walking the tree right-first - I was actually fairly surprised core clojure doesn't provide Reversible for ArrayLists already.
Patch taken from https://github.com/glenjamin/data.int-map/tree/rseq attached, benchmarks below:
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