Cleanup: Move RNA path functions into own C++ file

Adds `rna_path.cc` and `RNA_path.h`.

`rna_access.c` is a quite big file, which makes it rather hard and
inconvenient to navigate. RNA path functions form a nicely coherent unit
that can stand well on it's own, so it makes sense to split them off to
mitigate the problem. Moreover, I was looking into refactoring the quite
convoluted/overloaded `rna_path_parse()`, and found that some C++
features may help greatly with that. So having that code compile in C++
would be helpful to attempt that.

Differential Revision: https://developer.blender.org/D15540

Reviewed by: Brecht Van Lommel, Campbell Barton, Bastien Montagne

source/blender/editors/animation/anim_ipo_utils.c

@@ -22,6 +22,7 @@
 #include "DNA_anim_types.h"
 
 #include "RNA_access.h"
+#include "RNA_path.h"
 #include "RNA_prototypes.h"
 
 #include "ED_anim_api.h"