This adds a first version of the overall linearization interface, which given
a DepGraph constructs a good linearization, by incrementally including good
candidate sets (found using AncestorCandidateFinder and SearchCandidateFinder).
This primarily adds the DepGraph class, which encapsulates precomputed
ancestor/descendant information for a given transaction cluster, with a
number of utility features (inspectors for set feerates, computing
reduced parents/children, adding transactions, adding dependencies), which
will become needed in future commits.
