Support for nullable value types

[stephenberry]

This adds support for optional like types that cannot provide an operator bool() because that operator is reserved for some other action. This adds a nullable_value_t concept that only checks for value() and has_value().

[scalpel4k]

Nice, this will do the trick for me. Should be mentioned somewhere in the docs, I think.

thanks a lot!

[scalpel4k]

I've tested it with our code base and it works as intended

[stephenberry]

Great! I'm about to merge this. I added support for reading nullable value types as well, as long as reset() and emplace() methods are provided. If you don't have these methods on your type then you can just keep using your custom parse specialization.