Redesign API using policies

[Journeyman1337]

I think a policy class would be great to allow users to customize how the optional determines if the value is existing. This would allow users to customize this behavior while still using the same unified interface. This is similar to how allocators work in the stl. If its set to void, it can use std::optional behind the scenes instead. could do this by default.

template<typename TYPE, TYPE VALUE>
struct ValueSentinel final
{
   bool has_value(const TYPE& value)
   {
      return value != VALUE;
   }
};

template<typename TYPE, TYPE VALUE>
struct GreaterThanSentinel final
{
   bool has_value(const TYPE& value)
   {
      return value > VALUE;
   }
};

template<typename TYPE, typename SENTINEL = void>
Markable;

template<typename TYPE, typename SENTINEL>
Markable<TYPE, SENTINEL>
{
   // with [[no_unique_address]] the compiler is smart enough to optimize sentinel away if its an empty struct.
    [[no_unique_address]] SENTINEL _sentinel;
    TYPE _value;
    
    bool has_value()
    {
        return _sentinel.has_value(this->_value);
    }
};

// specialization for no sentinel
template<typename TYPE>
Markable<TYPE, void>
{
    std::optional<TYPE> value_o = std::nullopt;
    
    bool has_value()
    {
        value_o.has_value();
    }
};