create module so that RUSTC_LOG can filter to just proof trees

compiler/rustc_trait_selection/src/solve/inspect/dump.rs

@@ -0,0 +1,5 @@
+use rustc_middle::traits::solve::inspect::GoalEvaluation;
+
+pub fn print_tree(tree: &GoalEvaluation<'_>) {
+    debug!(?tree);
+}