This commits introduces support in DepGraph for the transaction positions to be
non-continuous. Specifically, it adds:
* DepGraph::RemoveTransactions which removes 0 or more positions from a DepGraph.
* DepGraph::Positions() to get a set of which positions are in use.
* DepGraph::PositionRange() to get the highest used position in a DepGraph + 1.
In addition, it extends the DepGraphFormatter format to support holes in a
compatible way (it serializes non-holey DepGraphs identically to the old code,
and deserializes them the same way)
This introduces a bespoke fuzzing-focused serialization format for DepGraphs,
and then tests that this format can represent any graph, roundtrips, and then
uses that to test the correctness of DepGraph itself.
This forms the basis for future fuzz tests that need to work with interesting
graphs.
