bugfix: Lua backtrace dumps upon uncaught Lua exceptions did not work with the standard Lua 5.1 interpreter when the backtrace was deeper than 22 levels. now we just dump the top 22 levels in the backtrace for simplicity. also optimized the Lua string concatenation operations when constructing the backtrace string.
change: we now limit the number of nested coroutines in the backtrace dump for uncaught Lua exceptions by 5.