Implement "recursive" version of existing operators

[bartop]

"Deep" versions of algorithms would work for nested enumerables without breaking this nested hierarchy. For example:

const auto input = { (std::vector{1, 2}), (std::vector{3, 4, 5}), (std::vector{6}) };
const auto output1 = input | deep_filter([](const auto &i){ return i >=2; });
// output1 is now {{2}, {3, 4, 5}, {6}}

const auto output2 = input | deep_filter([](const auto &i){ return i < 6; });
// output2 is now {{1, 2}, {3, 4, 5}, {}}

I will gladly disscuss pros and cons of keeping the empty containers inside hierachy.

Our TODO list is:

• transformed
• sorted
• filtered
• flattened