Problem:
- The standard does not provide a function object that calls unary `operator+`.
  This can be useful for conventional conversions, for example an overloaded
  `operator+` that converts an scoped enumeration to its underlying type.

Solution:
- Add `unary_plus` for this purpose.
- `unary_plus<void>` deduces its argument and provides `is_transparent` in
  accordance with standard practice.