Function open_os 

pub fn open_os(state: &mut LuaState) -> Result<usize, LuaError>

C: LUAMOD_API int luaopen_os(lua_State *L)

Opens the os library: creates a new table populated with OS_LIB and leaves it on the stack.

PORT NOTE: register_lib is the Rust equivalent of luaL_newlib; it creates a fresh table, fills it from the (name, fn) pair slice, and pushes it.