Tracepoints get codegenerated to a rts call

* (Tick TracepointTick) in core get converted to (StgTracepoint) in Stg
* (StgTracepoint)s get codeGenerated into a call to the rts function
  "pushTracepoint"

the rts function "pushTracepoint" is yet to be implemented so linking
will fail if -backtrace is used