This PR has two small contributions:
- It adds another phase to the timer for concrectization, "construct_specs", to actually see the time the concretizer spends interpreting the `clingo` output to build the Python object for a concretized spec.
- It adds the method `Solver.solve_with_stats` to expose the timers that were already in the concretizer to the public solver API. `Solver.solve` just becomes a special case of `Solver.solve_with_stats` that throws away the timing output (which is what it was already doing).
These changes will make it easier to benchmark concretizer performance and provide a more complete picture of the time spent in the concretizer by including the time spent interpreting clingo output.
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