…ressions

Drop support for reference-wrappers as the object expression (LWG2336), as it considerably complicates the already complicated code for no perceived benefit (pointers would be enough)

…dable

Make the return types of their invoke expressions explicit
Replace nested function types with helper function objects (no spirals)

Reduce the number of overloads and use of SFINAE.
Drop redundant overloads for known return types.
Reimplement `invoke_r` to tolerate overloaded comma operators.

Conflicts:
	hpx/lcos/local/detail/invoke_when_ready.hpp
	hpx/util/invoke.hpp

Reimplement `invoke_fused_r` to tolerate overloaded comma operators.

Conflicts:
	hpx/lcos/local/detail/invoke_when_ready.hpp