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
$ hg init
$ echo a > a
$ hg ci -Am t
adding a
$ hg mv a b
$ hg ci -Am t1
$ hg debugrename b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3
$ hg mv b a
$ hg ci -Am t2
$ hg debugrename a
a renamed from b:37d9b5d994eab34eda9c16b195ace52c7b129980
$ hg debugrename --rev 1 b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3