Add a way to generate tracing events programmatically

Add a way to generate tracing events programmatically

added:

 primop  TraceEventOp "traceEvent#" GenPrimOp
   Addr# -> State# s -> State# s
   { Emits an event via the RTS tracing framework.  The contents
     of the event is the zero-terminated byte string passed as the first
     argument.  The event will be emitted either to the .eventlog file,
     or to stderr, depending on the runtime RTS flags. }

and added the required RTS functionality to support it.  Also a bit of
refactoring in the RTS tracing code.