Implement `with` function

[quoll]

To implement with we want a Connection/Database wrapper that:

• Captures the underlying graph
• Creates an in-memory graph
• resolves against the underlying graph concatenated with the in-memory graph (filtering for uniqueness)
• accepts transactions against the in-memory graph
• records deletions for the in-memory graph, to filter out the underlying graph

[quoll]

I was expecting to return everything from the underlying graph, and then filter what was found in the in-memory graph. But this is backward. The in-memory graph can be used as a fast filter against the underlying graph. Similarly, a record of deletions can be kept just like an assertion graph, and used for filtering.

This means that deletions should:

• check if the statement appears in the in-memory graph. If so, then remove from there.
• if not in the in-memory graph, then check if the underlying graph contains that statement. If not, then ignore.
• if the underlying graph contains the statement, then keep in a deletion graph.

Additions should:

• check if the statement appears in the underlying graph. -- if so, then check the deletion graph. If not there, then exit. If there, then remove. -- if not, then check the in-memory graph. If not there then add. If there, then check in the deletion graph. If in the deletion graph, then remove, otherwise exit.