Added a 3D rotating cube example

[maximecb]

It took me a while, but I've been working on adding support for float to the C compiler, along with typedefs. That has been sufficient to implement a basic 3D math library:

https://github.com/maximecb/uvm/blob/main/ncc/include/uvm/3dmath.h

Today, I added a 3D rotating cube example:
297bf07

As you can see, there's some caveats. Currently, arrays can't be local variables on the stack, so this code example uses many global variables as a short-term workaround. Still, a good first step I think :)