Impact of C++20 lambdas on our code

TL;DR: uninteresting until we depend on C++20

C++20 added more feature to lambda functions:

1. optional template-initializer (__cpp_generic_lambdas >= 201707L)
2. pack-expansion in init captures (__cpp_init_captures >= 201803L)
3. lambdas can appear in unevaluated contexts:

   std::unique_ptr<FILE, decltype([](FILE *f) { fclose(f); })> file(fopen(~~~));
   

4. stateless ones are default-constructible and assignable

There's nothing we can do with these until we depend on C++20. When we do, these features will be useful in the implementation of Qt; I don't see an impact on our APIs atm.