Flush the debug trace file after each dbg!().

When debugging code built with panic=about, the debug trace buffer is
not flushed on a panic and important messages are lost.

This does slow down debug tracing a bit, but this is still better than
unbuffered or line buffered I/O since the dbg!() macro can write out
many lines like a whole function.

lib/cretonne/src/dbg.rs

@@ -63,7 +63,11 @@ thread_local! {
 ///
 /// This is for use by the `dbg!` macro.
 pub fn writeln_with_format_args(args: fmt::Arguments) -> io::Result<()> {
-    WRITER.with(|rc| writeln!(*rc.borrow_mut(), "{}", args))
+    WRITER.with(|rc| {
+        let mut w = rc.borrow_mut();
+        writeln!(*w, "{}", args)?;
+        w.flush()
+    })
 }
 
 /// Open the tracing file for the current thread.