[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)
remove space
simple test for get_device_pixel_ratio
add docs
add glob, globfree in musl; 1.23.3
Respect --llvm-opts when performing --llvm-lto=2
add error checking
Implement targetX and targetY fields for mouse and touch events, #2751. Bump version to 1.23.4 to rebuild cache since src/struct_info.json changes.
Add test for previous commit.
Support --profiling in addition to -profiling, following the 'two dashes for words, one dash for letters convention.' Fixes #2679
Update all instances in code to use '--profiling' instead of '-profiling'. Test in other.test_simplify_ifs that both forms are still valid.
Fix odd char formatting issue caused by saving the code file in previous commit.
Add support for all possible 7-bit ascii characters in filenames for --preload-file, notably ', " and @.
Document the valid character set for packaging files. Fix minor syntax in example for better form.
Merge pull request #2765 from juj/support_ascii_chars_in_files

Support ascii chars in files.
move get/set_element_css_size to html5.h