Export Solver and Theory Statistics

[ishehadeh]

Addresses #18.

This PR adds a trait, SnapshotStatistics, and a collection of *StatsSnapshot data structures which generally mirror the various Stats data structures for each reasoner as well as the main Solver struct.

I chose to define new stat types because the existing reasoners::*::Stat types seemed particularly suited to recording statistics, and didn't include all the information printed with the print_stats() functions. The *StatsSnapshot types are intended to have a field corresponding to each line from print_stats() , and generally be simple, static structures which are easy to export and reason about. These structures also implement serde traits, when enabled.

The trait has a straightforward definition, just returning some structure with a few basic constraints:

pub trait SnapshotStatistics {
    type Stats: Sized + std::fmt::Debug;

    fn snapshot_statistics(&self) -> Self::Stats;
}

Each component implementing Theory now implements SnapshotStatistics, with it's own *StatsSnapshot as Stats.

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As an example (and test) this PR also adds an option to export the aries-sat's solver stats as JSON.