Where clauses for more expressive bounds

Add `where` clauses, which provide a more expressive means of specifying trait parameter bounds. A `where` clause comes after a declaration of a generic item (e.g., an impl or struct definition) and specifies a list of bounds that must be proven once precise values are known for the type parameters in question.

Main benefits:

- Express bounds on types derived from type parameters, e.g. `Option<T>` where `T` is a type parameter.
- Permits true multiple dispatch.

[Rendered view.](https://github.com/nikomatsakis/rfcs/blob/where-clauses/active/0000-where.md)
Looking at this a bit more, I'm confused. The trait coherence rules documented above say that you can't implement the same trait twice on a given type, even if the type parameters to the trait differ. But here you're defining multiple `Add` implementations on the single type `()`. Isn't that a violation of the same coherence rule that prevented `Add<Complex, Complex>` + `Add<int, Complex>` on `Complex`?

And if you intended to relax the coherence rules to allow this, then what's the point on defining the trait on `(L,R)` to begin with? You could just implement it directly on `()`.

The only difference in the impl on `()` seems to be the fact that it's implemented in terms of type parameters, but I don't see why that should make a difference. And if it is somehow special, then what is the type of the expression `<() as Add>::add`?