Add a free function to compute the distance between two shapes.

cc #28.
Rename `dir` to `vel` on the arguments of TOI computation functions.

Those are velocities of movement, not just directions.
Remove the type parameter `I` from the `Compound` shape.

This was mostly an implementation detail for the `Volumetric` trait.
Now, it is hidden behind a `Any` ref to avoid the need of propagating it on all the rest of the
API unlrelated to inertia tensor computation (that's both ugly and annoying).