Trace to stderr, not stdout

This would interfer with stdio communications.

src/preserves.nim

@@ -9,8 +9,7 @@ from std/strutils import parseEnum
 import ./preserves/private/dot
 
 when defined(tracePreserves):
-  template trace(args: varargs[untyped]) =
-    echo args
+  template trace(args: varargs[untyped]) = stderr.writeLine(args)
 else:
   template trace(args: varargs[untyped]) = discard