Summary:
- The `Edges` data structure now handles common ops, including providing an iterator over edges whose "other" vertex is know.
- This should improve performance on dense_expand tests in the harness without other side-effects.
- query::plan::Expand operator modified not to check for existing-node stuff since that now gets handled by the `Edges` data structure.
- `Edges::Iterator` implemented only for const iterators since that suffices for now. Can implement non-const if the need arrises.
Reviewers: buda, mislav.bradac, teon.banek
Reviewed By: teon.banek
Subscribers: pullbot
Differential Revision: https://phabricator.memgraph.io/D763
