GSL.util: Utilities

• finally // finally(f) makes a final_action{f} with a destructor that invokes f
• narrow_cast // narrow_cast<T>(x) is static_cast<T>(x)
• narrow // narrow<T>(x) is static_cast<T>(x) if static_cast<T>(x) == x or it throws narrowing_error
• [[implicit]] // "Marker" to put on single-argument constructors to explicitly make them non-explicit.
• move_owner // p = move_owner(q) means p = q but ???