* Tentatively fix conflict between cstdfloat.hpp and multiprecision.
* Add missing struct to new code path.
* Reconfigure again for cstdfloat.hpp interop.
* Add missing #includes to float128.hpp.
Make constexpr __float128 arithmetic tests behave the same way, irrespective of whether numberic_limits is specialized for that type.
* Make tests conditional on BOOST_HAS_FLOAT128
Add tests for float128 constexpr.
Fix existing code to support constexpr-ness in more contexts via std::is_constant_evaluated or equivalent.
Change enable_if to enable_if_c in number.hpp in the (possibly vainglorious) hope of fixing some clang-apple failures.