Add eff to scala community build

[benhutchison]

Hi @etorreborre, both myself personally and REA are now building on top of Eff as a trusted platform.

One way we can strengthen the stability off Eff would be to include it in the Scala community build, so we'll know if future scala versions break it.

I work with Seth as Scala Swing module maintainer already, I'd be happy to volunteer as "build maintainer" for an Eff build.

I can send a PR to add Eff, just wanted to checkin with you first..?

[etorreborre]

(back from vacations now): I definitely approve.

[benhutchison]

scala/community-build#709

[benhutchison]

Seth asked if we could move the Eff benchmarks out of coreJVM so they aren't running in the community build, and so it doesnt depend on scalameter.

Moving https://github.com/atnos-org/eff/tree/master/jvm/src/test/scala/org/atnos/benchmark to root project seems to work...?

[benhutchison]

Done