Add variadic_helpers

Add variadic_helpers.hpp
* `get_type<i, Types...>`, gets a type by index
* `get_value<i>(values...)`, gets an argument by index
* `for_each_type<Types...>(lambda)`, visits all types
* `for_each_value(lambda, Types&& values...)`, visits all values

`for_each_type` and `for_each_value` will invoke a callable with the current index and current type/value. 