This makes a number of changes:
* Remove the dependency on marshal by using string.dump and loadstring.
* Use lua_tolstring rather than having Lua functions pass string lengths to C++.
* Move lua_api/l_async_events.* to cpp_api/s_async.*, where it belongs.
* Make AsyncWorkerThread a child of ScriptApiBase, this removes some duplicate functionality.
* Don't wait for async threads to shut down. (Is this safe? Might result in corruption if the thread is writing to a file.)
* Pop more unused items from the stack
* Code style fixes
* Other misc changes
On the lua side, notably minetest.env:<function>(<args>) should now
be replaced by minetest.<function>(<args>).
The old way is and will stay supported for a long time.
Also:
Update and clean up lua_api.txt (by celeron55)
Move EnvRef to lua and remove add_rat and add_firefly (by kahrl)
Add separate src/util/CMakeLists.txt, other minor fixes (by kahrl)
