Add a basic memory profiler, invoked with -m.
Rename |Profiler| as |TimeProfiler|.

And likewise for |ProfilerChan|, |profiler_chan|, and so on.  This
contrasts nicely with the newly added |MemoryProfiler|.
Merge pull request #2720 from nnethercote/memprof

Add a basic memory profiler, invoked with -m. r=pcwalton.