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
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