rust-hgpath: add HgPath and HgPathBuf structs to encapsulate handling of paths This change is a direct consequence of this discussion on the mailing list: https://www.mercurial-scm.org/pipermail/mercurial-devel/2019-August/133574.html The implementations of `HgPath` and `HgPathBuf` are, for the most part, taken directly from `OsStr` and `OsString` respectively from the standard library. What this change does *not* yet do is implement the Windows MBCS to WTF8 conversion, but it lays the basis for a very flexible interface for paths. Differential Revision: https://phab.mercurial-scm.org/D6773