Generate physx-safe on top of physx-sys

[NiklasNummelin]

Is your feature request related to a problem? Please describe.
As we now have ported pxbind to Rust it would be easier to make another backend to generate a, most likely unergonomic, safe layer around physx-sys. This would let physx-rs focus on the ergonomics while leaving the unsafe code to this generated layer.

Describe the solution you'd like
Build a backend to pxbind that generates code for a physx-safe crate that hides all, or most of the, unsafe interaction with physx-sys.

Related to: #3