Remove markers and replace with a default rule for unused type/lifetime parameters

Inspired by @edwardw's PR mozilla/rust#12828, this RFC proposes a small change to the behavior of variance inference if a type or lifetime parameter is unused. The goal is to more accurately capture user's intended behavior.
I was really worried about this, but now I like the idea, seeing how `ContravariantType` and `CovariantLifetime` can be emulated.