approaching good code regarded apply_tuple/bind
more cleanup, added xtm::curry
tuple utilities & improvements

 - tuple_cat_t
 - placeholder_list_t
 - curried_bindings_t

 rewrote xtm::curry to use these for cleaner code
updates to tuple & function_traits

 - function_traits gets a way to automagically remove references
 - xtm::bind now reduces binds of binds to a single resultant bind