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.

Do we think we'll want nested/tree profiling? That's something that we did on Stash and it was quite useful.