repository: define manifest interfaces The long march towards declaring interfaces for repository primitives continues. This commit essentially defines interfaces based on the following types: * manifest.manifestdict -> imanifestdict * manifest.manifestlog -> imanifestlog * manifest.memmanifestctx -> imanifestrevisionwritable * manifest.manifestctx -> imanifestrevisionstored * manifest.memtreemanifestctx -> imanifestrevisionwritable * manifest.treemanifestctx -> imanifestrevisionstored * util.dirs -> idirs The interfaces are thoroughly documented. Their documentation is now better than the documentation in manifest.py in many cases. With the exception of util.dirs, classes have been annotated with their interfaces. (I didn't feel like util.dirs needed the proper interface treatment.) Tests have been added demonstrating that all classes and instances conform to their interfaces. This work was much easier than filelogs. That's because Durham did an excellent job formalizing the manifest API a while back. There are still some minor kludges with the interfaces that should probably be addressed. But the primary goal with interface declarations is getting something established. Once we have an interface, we can modify it later easily enough. Differential Revision: https://phab.mercurial-scm.org/D3869