Combine serveral ConvertibleTo

[yhmtsai]

We have ConvertibleTo and usually for each precision we support.
Previously, we use the next_precision and macro condition to support them properly.
Alternatively, we can merge them together and use template such that it inherit from all ConvertibleTo<Class<Precision, Rest...>>

Also, we can use mixin such that it use the template implementation automatically. (EnableConvertibleTo)
However, the mixin using the template member function does not implicit instantiate the function.

Some direction in my mind

• only combine the inheritance: we will still have convert_to(next_precision) with macro
• use mixin with template: need to have macro for instantiation
  • alternatively, use runtime dispatch in internal function, such that they are implicitly instantiated (I somehow like this way, but I also feel making some unused internal function is a bit weird)
• use mixin with variant/runtime dispatch: similar to the above but merge runtime dispatch without template. However, it makes the result type to common type (or variant) and then cast back to the result type.
• any idea?

[MarcelKoch]

nit: I think the type could better be called conversion_types.

[MarcelKoch]

I don't think we should have a *_impl function on the public interface. Either is should be renamed, or made private.

[yhmtsai]

yes, it should be in private although it will need all EnableConvertibleTo<*> to be friend of all class