Extract object_mixin_data management from object_mutation

[iboB]

object::clear_mixin_data is noexcept, but relies on an noexcept(false) destructor. It should never throw in the given case, but still the compiler will generate calls to std::terminate there. It is possible to create a class basic_object_mutation which provides alloc/dealloc/destroy functions for mixin data. I can have a noexcept destructor and can be used by both object_mutation and clear_mixin_data.