Run showValue on sparse list values

This takes us from 70% of the time and lots of memory in spent showValue
on simple input to much more negligible time and space.

src/GHC/RTS/Events/Analyze/Reports/Timed.hs

@@ -123,7 +123,7 @@ writeReport' report h =
 
     reportLine :: ReportLine -> [String]
     reportLine ReportLineData{..} =
-      lineHeader : map showValue (unsparse 0 lineValues)
+      lineHeader : unsparse "0.00" (Map.map showValue lineValues)
 
     showValue :: Double -> String
     showValue = printf "%0.2f"