function_traits doesn't support qualified function types

Add the moment `function_traits` works for `void () noexcept`, but not for `void () const` or `void () &`.

If we fix that, it would be a good opportunity to also add
```
{
template struct list {};
using arguments = list;
}
```
(on C++11) which can be further manipulated with Mp11. (The `argN_type` typedefs aren't generic.)