Update to the last rust-nightly.

Version of rustc: 0.13.0-nightly (42deaa5e4 2014-12-16 17:51:23 +0000).

This is a quick fix that introduces some backward incompatibilities: it assumes that `N` is always `Copy` (and `Clone`). If we do not make this assumption, we have to be careful and restrict the amount of cloning to a minimum as it may induce some costly memory operations (e.g. allocation). This patch is not that careful because doing so requires a lot more work (perhaps introducing new traits for in-place operations like: `mul_to(left: &Vec3<N>, right: &Vec3<N>, out: &Vec3<N>)`. I will create an issue about this as soon as I have the time to develop this.

The two salient backward incompatibilities brought by this patch are:

1. operator overloading will not work any more if the scalar type is not `Copy` (e.g. you won't be able to add two `Vec3<DVec<f32>>`).
2. QR decomposition (and the QR method for eigenvector extraction) won't work for `DMat` any more.