This reverts commit 362ef5f6.

Stack tracebacks couldn't be generated in LuaError::LuaError anyway and this
caused a second, empty traceback in most cases.  In cases where there wasn't
annother traceback the stack had already unwound and the traceback was empty.

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)