[tracing] Initial 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.
[tracing] Integrate the tracing stuff.
[tracing] Instrument dlmalloc.c

These changes compile out entirely if tracing is not enabled.
[trace] Point to the server's git repository.
Merge pull request #2622 from waywardmonkeys/trace

Initial tracing API (initially does memory profiling and analysis)