Formalize conversion with generic 'from'/'into' free functions

[elBoberido]

Brief feature description

While the conversion operator can be implemented for classes, there is no such possibility for enums. To have a consistency in the code base there should be a convention on how to do it. The proposal presents from/into free functions.

Detailed information

Similar to the Rust From/Into trait, there are templated from/into free functions. A developer has to implement only from and into will automatically be available.

This is the implementation in convert.hpp

template <typename F, typename T>
constexpr T from(const F) noexcept
{
    static_assert(always_false_v<F> && always_false_v<T>, "Conversion for the specified types is not implemented!");
}

template <typename T, typename F>
constexpr T into(const F e) noexcept
{
    return from<F, T>(e);
}

The developer can then specialize the from free function for a given enum

enum class Foo
{
    Hypnotoad
};

enum class Bar
{
    Glory
};

template <>
Bar from<Foo, Bar>(Foo e) noexcept
{
    switch(e) {
        case Foo::Hypnotoad:
            return Bar::Glory;
    }
}

On the caller side the developer can just use auto barEnumValue = cxx::into<Bar>(Foo::Hypnotode);