Add ReferenceTrimmer and remove unused references

[dfederm]

Add ReferenceTrimmer to enable detection of unnecessary project, package, and assembly references during build. Remove all unused references identified by ReferenceTrimmer.

Disclosure: I own ReferenceTrimmer, but I do think it will be helpful to this repo, as evidenced by the unused references it was able to identify. Adding it to the repo will help keep the references clean.

[mgravell]

Interesting tool, thanks. I'm a little cautious of additional build-tools, but since I also own DapperAOT and a bunch of other build-time tools, it is probably hypocritical for me to get too paranoid about that! I've poked it a bit to deal with merge-conflicts, update to latest, etc, but if it goes green: great! appreciated