Continue parser cleanups

Prepare to split the interface of sg_platf in two: one public
interface (include/simgrid/platf.h) will contain the function
available to the user code to create elements while another interface
(src/include/simgrid/platf_interface.h) will contain what internal
modules need to declare callbacks on the element creation events.

For now, the files are still very empty, but I try to not break stuff
even further, so I commit soon.