Use ujson if available in slice_trace.py
Use ujson if available in slice_trace.py

Summary: This script is meant for working on large traces, so it can be slow. Use a faster C JSON module if available to improve performance.

Reviewed By: swgillespie

fbshipit-source-id: ba4a39c413969d6d6c041fbabf532cfe7f6383de

JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck