So fastutil is now part of Solr's distribution (because the stats component uses hyperloglog library, which in turn requires fastutil). I looked at the actual uses of fastutil and only java-hll uses it (and only a few classes).
I've created a fork that uses HPPC instead (also randomized all tests, they pass). Since it's a relatively simple package I think it could be forked and imported into Solr's codebase entirely. I'd make a pull request but I see Hoss also created a few comments/ PRs and none of them received any attention; the project seems to be stale or dead?