remove subtyping relationship for safe/unsafe fns and replace with coercion

[nikomatsakis]

Currently, fn is a subtype of unsafe fn. This should be a coercion relationship, pursuant to our strategy of limiting subtyping to region relationships. This also implies that one cannot implement an unsafe trait method with a safe fn, which is useful for solving the "trusted iterator length" problem.

[nikomatsakis]

triage: P-backcompat-lang (1.0 beta)

[nikomatsakis]

This is not super important, I think we could live with it staying the way it is. But I've got a branch already that seems to be mostly building.