Remove bare function types, replace with inference

[nikomatsakis]

We should remove the types for bare functions altogether and just infer the appropriate closure type when referring to an item.

[nikomatsakis]

Deferring to 0.5.

[nikomatsakis]

I've been working on this, current patch is blocked on #2263.

[pnkfelix]

The goal of this bug is not clear to me. If the goal is to enable the use of inference in more contexts, then by all means, lets do that. But if the goal is to remove the option of writing down an explicit type, then I would probably not be in favor of it. (The ability to declare types explicitly has been a useful tool for me in narrowing down cases where type inference signals a problem.)

[pnkfelix]

In any case, this is not something that should block 0.6.

[nikomatsakis]

Plans have changed somewhat. We are keeping the bare fn types. I think this issue is no longer relevant.