Add support for using `std::enable_if` in `value_traits` specializations

[boris-kolpackov]

Motivating example:

template <typename EnumType>
class value_traits<EnumType, id_string, typename std::enable_if<std::is_enum<EnumType>::value>::type>
{
public:
    using query_type = EnumType;

    static void set_value(
            EnumType& value,
            details::buffer const& buffer,
            std::size_t n,
            bool isNull)
    {
        // deserialize database string to any enum value
    }

    static void set_image(
            details::buffer& buffer,
            std::size_t& n,
            bool& isNull,
            EnumType const& value)
    {
        // Serialize any enum value to database string
    }
};

To make this work would require changing the value_traits template from:

template <typename T, database_type_id ID>
class value_traits: public select_traits<T, ID>::type
{
};

To:

template <typename T, database_type_id ID, typename = void>
class value_traits: public select_traits<T, ID>::type
{
};

Which will break all the existing code that expects the old template.

So while the idea is good, we need to think how we can implement it without break backwards compatibility.

[boris-kolpackov]

We've tried a couple of ideas to achieve the same without changes, but none of them work:

template <typename EnumType>
class value_traits<typename std::enable_if<std::is_enum<EnumType>::value, EnumType>::type, id_string>
{
  ...
};

class dummy {};

template <typename EnumType>
class value_traits<EnumType, id_string>: std::enable_if<std::is_enum<EnumType>::value, dummy>::type
{
  ...
};

template <typename EnumType>
class value_traits<EnumType, enable_value_traits_if<std::is_enum<EnumType>::value, id_string>::id>
{
};

What does work is a C++20 concepts-based solution:

#include <type_traits>

template <typename T> concept Enumeration = std::is_enum_v<T>;

template <typename EnumType> requires Enumeration<EnumType>
class value_traits<EnumType, id_string>
{
   ...
};

Also, one nasty thing about adding the extra Enable template argument just to allow the use of std::enable_id is that symbols for every value_traits function will now include it.

At the moment keeping things as is and using C++20 concepts feels like the best solution.