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
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