Add the HACD algorithm + implement Volumetric for Convex.
Improve dramatically the performances of hacd using some algorithmic tricks.
Make the 3d convex hull implementation more robust to degenerate inputs.

Instead of failing, it will return a degenerate triangle mesh.
Make hacd support meshes with split index buffers properly.
Some improvement on the hacd algorithm.
Robustify the convex hull computation using a PCA + improve the hacd cost function.

This also fixes various area, volume, and mass properties computations.
Update to the last rust-nightly.

Version of rustc: 0.12.0-pre-nightly (01ec6fab2 2014-08-18 00:46:10 +0000).
Convex hull 3d: is no horizon is found, just stop the algorithm.

Not finding an horizon is a sign of a very bad problem, and most of the time it will crash with a
"affinely dependent facet" error. For the moment, we will just abort the algoritm at this point,
and return the current result.

A proper fix will have to be found though.