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.
It should be implicit - otherwise for any function taking an "any" you have to explicitly construct one. I see the issue with your serialization work - let's see if we can't find a better solution.