Function to_lua_string 

pub fn to_lua_string(
    state: &mut LuaState,
    idx: i32,
) -> Result<Vec<u8>, LuaError>

Convert the value at idx to a byte-string representation (using __tostring if available) and push it onto the stack.

C: LUALIB_API const char *luaL_tolstring(lua_State *L, int idx, size_t *len)