Means to apply functions on tuples as if they were arg lists.
Revert "Functor implementation for ftl::function."

This reverts commit 7d77cb3c7e165246260c90e17621f241559a07a4.
Revert "Major restructuring of ftl::function (and dependants)."

This reverts commit a141669c076cb13da050fc0f78461bce949a58ce.