Unique function

[K-ballo]

This patch introduces unique_function (as an implementation detail), a counterpart to function that is movable-only and can only be called once. By using it, we can finally support movable-only arguments to actions and as closures to executors.

Breaking changes:

• hpx::threads::thread_function_type was changed from thread_state_enum(thread_state_ex_enum) , now thread_function_sig, to unique_function<thread_function_sig>. Previous occurrences of HPX_STD_FUNCTION<thread_function_type> were replaced with just thread_function_type, and adjustments were made as appropriate to avoid copying them.
• Executors were changed to use unique_function<void()>, exposed as closure_type, instead of HPX_STD_FUNCTION<void()>. Adjustments were made as appropriate to avoid copying closures.

The implementation of unique_function currently incurs a lot of duplicated code that could be shared with function. Doing so, however, requires some drastic redesign of the virtual tables and how they are generated. It will nevertheless be provided by a future patch.

[StellarBot]

Tests Failed.
Refer to this link for build results: http://hermione.cct.lsu.edu:5632/job/hpx_pr_debug_gcc_44_boost_1_55_0/10/

[hkaiser]

The failing tests are unrelated to the given patch