python bindings for compressed graph access

We want to have some basic python bindings for accessing the compressed graph representation produced by WebGraph.

Edge-by-edge traversal round-tripping between python and java wouldn't be great performance-wise (yet still useful in various cases), but bindings for entire visits returning integer node identifiers would be quite usable as an alternative to relying on the REST API.

The current best way for doing python<->java bindings seems to be Py4J.

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Migrated from T1884 (view on Phabricator)