Replace hpx::function with hpx::util::function

[brycelelbach]

[reported by blelbach] [Trac time Fri Oct 28 23:58:52 2011] The first version of hpx::function should be replaced with the new hpx::function that heller wrote (currently lives in hpx::util::function). ATM, hpx::function is only used in a handful of examples and the AGAS symbolic naming layer.

[brycelelbach]

[comment by hkaiser] [Trac time Sat Oct 29 16:54:12 2011] This additionally has to

• remove the signature<> class from the actions hierarchy and
• remove the function virtual execute_function(); from the actions hierachy

[brycelelbach]

[comment by hkaiser] [Trac time Sat Feb 18 21:02:09 2012] fixed in rev. [7055]