Profile browser: several enhancements.

- UI to expand code views in profile
- Stacked histograms display
- Allow undefined values anywhere
- Show missing numeric values as empty strings

Signed-off-by: Mihai Budiu <[email protected]>

Author: Mihai Budiu

rust/template/ddlog_profiler/profiler_ui/ui.css

@@ -55,7 +55,7 @@ td.invocations {
 }
 
 .filename {
-    background: pink;
+    background: lavender;
 }
 
 .console {