Added a non-interfering serialization call to any_regular_t that calls operator<<(std::ostream&, T&) only when it is defined.

When it is not defined, the result is (eventually) a no-op.

Added a serializable_t concept type, and propagated it through the iomanip code, so an iomanip-able type now need only be serializable, not regular.

Added a new any_regular serialization test, and asserted that test/serialization still outputs as expected.
Any further thoughts here on allowing any_regular_t to be implicit, and still preventing a serializable_t -> any_regular_t -> serializable_t infinite loop? I'll keep chewing on it, but haven't come up with much just yet. Maybe a template specialization for any_regular_t in the iomanip code?