Remove `Collections` implementation and move `fn len` into `impl RWops`.

The `Collection` trait has been removed in accordance with the [std::collections reform](https://github.com/rust-lang/rust/issues/18424) and as far as I can see, there are no traits specifically offering a `fn len` anymore (for example, Vec now [implements it uniquely](http://doc.rust-lang.org/collections/vec/struct.Vec.html#method.len) as well).