ASV: add benchmarks to test formatting func in Styler.
The main time in rendering Styler is consumed by:

- formatting values for display
- adding cell styles via apply / applymap
- adding cell styles via set_td_classes

We have benchmarks for the latter 2 but not the first. Any future changes to the formatting function might be worth having benchmarks (I would use them :) )
Especially if we try to merge Styler formatting with the floatformatter for example. 

thanks