- `std::apply` is part of C++17 and the compiler wants to use it because
of ADL. Thus it is now necessary to escape it as `::apply`.
- remove some `using namespace std` remaining in the sources.
- fix simple warnings.
- add "Forges" tools, to wrap several operator with deferred
instanciation.
- add t-forge-algo to show how to enumerate several algorithms instances
from a common grammar and several alternative operators.
- add several missing className().
