Add a backtrace flag.
It doesn't do anything yet. It will be used to
tell GHC to simulate stack traces and dump them in the event of an error.
Add empty module BacktraceTypes.hs and register it with Cabal
Add the Tracepoint type (to BacktraceTypes.hs).
A tracepoint is very much like a SCC in profiling. It holds the name, module and location of the function.
Add new Unique for Tracepoints.

This is useful for fast comparison of tracepoints.