This patch implements the underlying mechanism for range adaptors. It does so based on http://wg21.link/p2387, even though that paper hasn't been adopted yet. In the future, if p2387 is adopted, it would suffice to rename `__bind_back` to `std::bind_back` and `__range_adaptor_closure` to `std::range_adaptor_closure` to implement that paper. REVIEW NOTE ----------------------------------- Once we agree on the direction, I will split it up into a few patches that add the required helpers and add tests for everything in those patches. The tests I have right now are the strict minimum I added to make sure things worked.

This patch implements the underlying mechanism for range adaptors. It does so based on http://wg21.link/p2387, even though that paper hasn't been adopted yet. In the future, if p2387 is adopted, it would suffice to rename `__bind_back` to `std::bind_back` and `__range_adaptor_closure` to `std::range_adaptor_closure` to implement that paper by the spec.

This patch implements the underlying mechanism for range adaptors. It does so based on http://wg21.link/p2387, even though that paper hasn't been adopted yet. In the future, if p2387 is adopted, it would suffice to rename `__bind_back` to `std::bind_back` and `__range_adaptor_closure` to `std::range_adaptor_closure` to implement that paper. REVIEW NOTE ----------------------------------- Once we agree on the direction, I will split it up into a few patches that add the required helpers and add tests for everything in those patches. The tests I have right now are the strict minimum I added to make sure things worked. by the spec.