Adds rna_path.cc and RNA_path.h. We could include the header in RNA_access.h for convenience, rather than including it in a bunch of files that already include the latter. But I think we should at least keep the path functions in a separate file.

Motivation
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 its 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. There are a number of improvements we could do to the RNA path code, and C++ seems to provide a number of helpful ways to do this. E.g. having paths as objects with safe interfaces, rather than mere character arrays seems like a good direction to go.