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.

Can this just be `state.putIfAbsent(k, new AtomicLong(0L)). addAndGet(n)`?