RFC: error conventions and syntactic support (including changes to macro syntax)

This RFC sets out a vision for refining error handling in Rust, in two
parts:

* A stronger set of conventions around `fail!` and `Result`
* Sugar to make working with `Result` highly ergonomic

In a nutshell, the proposal is to isolate uses of `fail!` to an extremely small
set of well-known methods (and assertion violations), with all other kinds of
errors going through `Result` for maximal flexibility. Since the main downside
of taking this extreme position is ergonomics, the RFC also proposes notation
for consuming `Result`s:

* Change macro invocation syntax from `macro_name!` to `@macro_name`.
* Use `foo!` for today's `foo.unwrap()`
* Use `foo?` for today's `try!(foo)`

While the two parts of this proposal reinforce each other, it's possible to
consider each of them separately.

[Rendered](https://github.com/aturon/rfcs/blob/error-conventions/active/0000-error-conventions-and-sugar.md)
FWIW, only the `Fn`, `FnMut` and `FnOnce` choice is 100% important here. Capturing by reference is just capturing a reference by value (the `ref |...| ...` syntax is sugar for capturing references), and `|ref x|` is not particularly relevant, it's the same was writing `|x| ... &x`; it's just taking a reference to one of the closure's arguments.

It may be possible to infer the most general closure trait based on how captured variables are used (try `Fn`, then `FnMut` and then `FnOnce` as a "last resort"), and thus sugar like this could be feasible without requiring a sigil soup.