Fix the generic wildcards of the FluentFutures.from methods.

This CL changes the generic arguments for the passed in iterables to the from methods so that they match the corresponding methods in Guavas Futures class. I've also added a (potentially controversial) compiler check class to make sure that these issues don't regress.
I guess they could, it really makes no difference I suppose. I figured keeping them as close to junit test methods as possible would be preferable.