add simple per-application run log file

[assertivist]

• big mess to get milliseconds in the log file prefixes loglines with date, time, and SDL ticks
• maybe should use SDL file operations instead of nice c++ streams
• no idea if printf is what we want to replace the stdout logging
• no idea if holding a file open will do bad things if the game crashes
• no testing anywhere but macos
• truncates file every time the app is opened (no rotation or keeping old files)

[assertivist]

added a 4k line library called "backward-cpp" to give us the bare minimum of stack unwinding. This will give us object/function/offset in the event of a crash, which should help us narrow down the last networking issues.