Generalize perf counters/timers

Timers are stored in a list, one per call, and counters are scalars updated on each call.

Provide a general underlying object ~ `PerfCounter`/`StatCounter`/`VariableStats`/`Trace`/similar that could be used to implement both behaviours + do on-update stats (count/min/max/mean/median).

Block developers could then use such `PerfCounter` to trace/log/aggregate values of interest, per iteration (e.g. (sub)problem size, max chain length, best sample energy - maybe even per branch, etc.)