Show build graph statistics in ghcide-bench (#2343)

* Show build graph statistics in ghcide-bench

This adds 5 new columns to the benchmark outputs:

- buildRulesBuilt   - for which the value didn't change
- buildRulesChanged - for which the value did change
- buildRulesVisited - for which the value was not even recomputed
- buildRulesTotal   - including the rules that were not visited in the last build
- buildEdges        - total number of edges in the build graph

* Fix build

* backwards compat.