[v3] implement LWG 2067 and new issues with constructors in <future>

[Jonathan Wakely]

On 2 November 2011 00:53, Jonathan Wakely wrote:
> The first is that packaged_task's template constructors should
> be restricted to prevent them from being chosen to copy a
> packaged_task object

While submitting that issue to the LWG chair I realised the constraint
should use decay<Fn> instead of remove_reference<Fn> so that it also
removes cv qualifiers.  I'll fix that tomorrow.