Initial draft of a tracing API.

This is used in conjunction with a collection & analysis server
to analyze memory usage and other aspects of a program running
under emscripten.
It is inside of an ``#ifdef __EMSCRIPTEN__`` ... and when you get to ``emscripten/trace.h``, you'll see that it is set up predicated on ``__EMSCRIPTEN_TRACING__`` to either actually do tracing or remove everything via the C preprocessor.  Seemed like the easiest way to keep everything clean.