Add some more metrics

[snowleopard]

This PR adds two time metrics:

• Cycle detection (time spent in Memo.add_dep_from_caller function)
• Generic digest computation (time spent in Digest.generic)

For the zero build of Dune, I get the following with --print-metrics:

69650/69650 computed/total nodes, 181578/181578 traversed/total edges
(2.19s total, 0.24s cycle detection, 0.06s digests, 24.7M heap words)