rust-core: extracted a revlog submodule This moves fundamental definitions from the top of the crate to the newly created `revlog` submodule and reexports them for easy compatibility. As we are about to add new features to this crate, we felt it will improve clarity, and moreso if `ancestors` and `dagops` would become submodules of `revlog`. Differential Revision: https://phab.mercurial-scm.org/D7782