base::lambda becomes just std::function and base::lambda_once becomes
base::unique_function - a move-only wrapper around std::function.
This is required because Visual C++ 2017 15.4.1 has a compiler bug
with static member variables of class templates, they may collide.
The std::function uses inheritance and virtual functions instead of
custom vtables done by static members of class templates used in
custom base::lambda implementation, so they work fine.
John Preston