Simplify detail/primitives.hpp.

* Use boost::mpl::identity everywhere instead of an own reimplementation.
* Do not include yes_no.hpp: clients which need it should include it
  themselves.
* Remove converter_policy_tag, which was inherited from but never queried for,
  i.e. it was practically useless.
* Remove unused struct aligned<N>.
* Remove unused function ptr_offset<Derived, Base>().