Use a union to store common_iterator's members

[tcbrindle]

The current implementation of common_iterator stores both the iterator and sentinel as direct members. Since only one of the members can be active at any time, this always wastes space.

P0896 has a note that common_iterator's members should be respecified in terms of std::variant. While we can't use variant (as it's C++17 only), we should use a simple tagged union to avoid making common_iterator twice as big as it needs to be.