A general Profiler data type, in the same style as Trace.

Needs to be rebased after topic/trace.

This is still pretty rudimentary, but I think it is probably ok to start with and we can add combinators as we go. 

The other useful thing to do would be add a constructor to support profiling to some standard metrics library.

Yeh, javalolz. If you can see a better way let me know, but I can't see anyway to avoid it.