Hide particle_collisions functions, replace #def.

particle_collisions/src/particle_collisions.(c, h)
	-Redefine `particle_collisions.c` functions as `static`, move
	the corresponding declarations from `particle_collisions.h` to
	`particle_collisions.c`, and remove `particle_collisions.h`,
	because none of the module's functions are really intended to be
	public.
	-Replace `#define` constant object-like macros with global
	`const` variables.