Make try! an extended version of std::try!

The only caveat to this is that you cannot use try! inside of a function
that return a Result to another function that expects a totally generic
error.

The primary case of this is Encodable/Decodable, which call into
user-specified methods, expecting an `E` back. In these (extremely
unusual) cases, you can use raw_try!.

Of note:

* Once Error lands in core (optimistic much?), Encodable/Decodable can
  be changed to expect an E: Error + FromError<E>.
* This works fine with concrete error types, since FromError maps things
  like IoError to IoError. The only reason it doesn't work with totally
  generic E is that we can't implement FromError<Box<Error>> for
  impl Error and FromError<E> for E.