rust: introduce Repo and Vfs types for filesystem abstraction This is similar to the corresponding Python classes. Repo represents a repository and knows the path to the `.hg` directory, the `store` directory, and the working directory. Separating these will enable supporting the share extension. A Vfs is created from a Repo for one of these three directories. It has filesystem access APIs that take a relative std::path::Path as a parameter. Differential Revision: https://phab.mercurial-scm.org/D9596