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)
This would also represent a change from pass-by-shared-reference to pass-by-value. Have you given it some thought whether there is a way to support both variants? For some types pass-by-shared-reference might be more efficient. With settling on tuples it seems to be even harder to get the best of both parameter passing styles.