Add new functions emscripten_log() in emscripten.h which allows printing out log messages with callstack information, and function emscripten_get_callstack(), which allows programmatically obtaining the current callstack.
Also give the ability to print out function parameters in the callstack with emscripten_get_callstack.
Update emscripten_log to work with the handwritten JS demangler. Update tests.
Move emscripten-source-map.min.js to src/ since it is a non-tests file needed by public use when user wants to emscripten_log with a C callstack. Remove redundant emscripten_demangle function.
Merge pull request #1635 from juj/emscripten_log

emscripten_log() and emscripten_get_callstack()