add std.debug.Trace.format

This makes it show up in some useful places; for example in the
self-hosted compiler we already print it now with
--debug-compile-errors.

lib/std/debug.zig

@@ -2023,5 +2023,22 @@ pub fn ConfigurableTrace(comptime size: usize, comptime stack_frame_count: usize
                 }) catch return;
             }
         }
+
+        pub fn format(
+            t: Trace,
+            comptime fmt: []const u8,
+            options: std.fmt.FormatOptions,
+            writer: anytype,
+        ) !void {
+            _ = fmt;
+            _ = options;
+            if (enabled) {
+                try writer.writeAll("\n");
+                t.dump();
+                try writer.writeAll("\n");
+            } else {
+                return writer.writeAll("(value tracing disabled)");
+            }
+        }
     };
 }