Add support for std::set serialization

Nobody has claimed this yet.

To serialize, iterate over the elements and write each one to the output stream. To deserialize, read the elements in and add each one to the set. `std::set` should take care of ordering correctly, since each object must have had a [comparison function](https://en.cppreference.com/w/cpp/named_req/Compare) to be inserted into the set.

Add unit tests to ensure serialization and deserialization works correctly.

Update `README` to include `std::set` support.

See [`array.hpp`](https://github.com/LukasJoswiak/draft/blob/master/include/types/array.hpp) for a template on how to serialize and deserialize.

Start with include/types/array.hpp to follow the existing serialization and deserialization pattern for a container. Add unit coverage for std::set round trips and update README to list std::set support. Done means sets serialize and deserialize correctly while preserving their ordering behavior.

Written by the indexing model from the issue text.