use wart remover

[johnynek]

we should turn on wart-remover: https://github.com/puffnfresh/wartremover to disable some aspects of scala that lead to bad errors or unsafe code.

Inferring Any, any2addString, Unit coercion are a few that look like they should definitely be on. I'd like to do more, but we may have some performance reasons for not turning on all the safety checks.