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
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