RigdBodyDesc can't be stored

The RigidBodyDesc accept a reference to the colliders https://github.com/rustsim/nphysics/blob/master/src/object/rigid_body.rs#L727

The problem is that due to the lifetime used there is not possible to store the RigidBodyDesc in the heap and move it around.

Since the RigidBody only exist inside the world, and I need to manipulate its parameters even out of the world, would be nice to be able to store them directly inside the descriptor.

Start with src/object/rigid_body.rs around line 727, where RigidBodyDesc references colliders, and trace how the descriptor is created and passed into the world. Determine an ownership design that allows the descriptor to be stored on the heap, moved, and used to manipulate parameters outside the world; done means this workflow is supported without the current lifetime restriction.

Written by the indexing model from the issue text.