Avoid panic in Itertools::format_with (#108)

When tracing is configured with multiple output layers, the argument to
`tracing::debug!` may be evaluated multiple times. Since
`Itertools::format_with` panics if `fmt` is called on it more than once,
use `to_string` to materialize the value into something safe to format
multiple times.

Author: jrray

src/solver/mod.rs

@@ -572,6 +572,7 @@ impl<D: DependencyProvider, RT: AsyncRuntime> Solver<D, RT> {
                         self.clauses.kinds[derived_from.to_usize()]
                             .display(&self.variable_map, self.provider()),
                     )))
+                    .to_string()
             );
             tracing::debug!("====");