[4.0.0] Require users to explicitly define StringType, etc.

[falbrechtskirchinger]

I'm in the process of debugging #3602. It seems that it would be beneficial if users were required to explicitly tell the library what types are StringTypes, etc.

Just because a type is constructible from a string doesn't make it a string type. This can lead to the compiler selecting (or trying to select and failing) the wrong from_json function.

Leaving this here to be fleshed out at a later date.