Add helper utilities for variadic templates
Variadic support for tuple. Adjusted other pieces that were relying on tuple's faux variadics.

Only the interface of tuple uses variadics, to make it possible to use variadics elsewhere. The actual implementation is still preprocessed, to keep a flat layout.
Variadic support for invoke
Variadic support for result_of
Variadic support for deferred_call
Variadic support for bind
Variadic support for function and unique_function
Variadic support for mem_fn
Variadic support for zip_iterator
Merge pull request #1306 from STEllAR-GROUP/variadics

First batch of variadic templates support