Expose tracebacks in safe API

[bobbens]

I use tracebacks when displaying warnings. However, to the best of my knowledge, luaL_traceback is not exposed in the "safe" API. In order to get backtraces, I had to wrap the unsafe luaL_traceback with create_c_function to be able to get the backtraces. Providing some sort of lua.traceback() or the likes that would do this could be useful instead of needing a custom implementation.

For reference, my unsafe implementation is below:

        #[allow(non_snake_case)]
        unsafe extern "C-unwind" fn traceback_wrap(L: *mut mlua::lua_State) -> i32 {
            unsafe {
                let msg = if mlua::ffi::lua_isstring(L, 1) != 0 {
                    mlua::ffi::lua_tostring(L, 1)
                } else {
                    std::ptr::null()
                };
                let level = if mlua::ffi::lua_isinteger(L, 2) != 0 {
                    mlua::ffi::lua_tointeger(L, 2)
                } else {
                    1
                };
                mlua::ffi::luaL_traceback(L, L, msg, level as i32);
            }
            1
        }
        let traceback = unsafe { lua.create_c_function(traceback_wrap)? };