New inference scheme

The inference scheme proposed in <http://smallcultfollowing.com/babysteps/blog/2014/07/09/an-experimental-new-type-inference-scheme-for-rust/>.

This is theoretically a [breaking-change]. It is possible that you may encounter type checking errors, particularly related to closures or functions with higher-ranked lifetimes or object types. Adding more explicit type annotations should help the problem. However, I have not been able to make an example that *actually* successfully compiles with the older scheme and fails with the newer scheme.

f? @pcwalton, @pnkfelix