* Adjust definition to make the default constructor `constexpr`.
  This permits use in classes that want lazy initialization and their
  own `constexpr` constructor, such as `mstd::mutex`.

* Add `get_no_init()` method to allow an explicit optimisation for
  paths that know they won be the first call (such as
  `mstd::mutex::unlock`).

* Add `destroy()` method to permit destruction of the contained object.
  (`SingletonPtr`'s destructor does not call its destructor - a cheat
  to omit destructors of static objects). Needed if using in a class
  that needs proper destruction.