Make Current measurement serializable

Note that this REALLY slows it down, since it forces the Region to be
Boxed.

It might make sense at some point to consider introducing a RegionBitmap
type. Maybe keep the original Region type -- for serializing -- but
cache a Bitmap version where perf matters more.

src/render.rs

@@ -304,6 +304,8 @@ impl Renderer for ColorTermRenderer {
 
         stdout.queue(PrintStyledContent(style(format!("z: {}", z)))).unwrap();
         for m in measurements {
+            // Measurements can be slow to compute
+            stdout.flush().unwrap();
             let meas_string = m.eval(state);
             stdout.queue(cursor::MoveDown(1)).unwrap();
             stdout.queue(cursor::MoveToColumn(1)).unwrap();