util/u_trace: Add json output

If we want to load the u_trace output somewhere for analysis it's much easier to deal with json than to parse strings.

CC: @anholt @llandwerlin

The goal is to have a script which allows easily see whether new optimization helped, how much, and where. E.g. a 0.5% total improvement in perf may be smeared through all renderpasses (and be a fluke), or be 50% win in a few of them.

WIP script I wrote and example of the output: Danil/mesa$6008

If you like the idea, where would be a good place to place the script for gather and analysis of the data?

Previously I did a more narrow analyzer - !15376, but decided to start from something more broadly useful.