Use fast pimpl idiom for our serialization classes

[Neverlord]

We are already using the pimpl idiom in a few places to give us more leeway for changing the implementation without breaking the ABI and to reduce compile times. However, there's a lot more classes where it would make sense to push implementation details out of the header. Of course forcing heap allocations onto all of these would be decremental to the performance. Hence, fast pimpl idiom. Isn't quite as flexible because the size of the implementation object is fixed but avoids the extra heap allocations.

The first set of classes where deploying the fast pimpl idiom makes sense are our serialization classes.

[Neverlord]

• config_value_writer
• config_value_reader
• json_builder
• json_reader
• json_writer
• binary_serializer
• binary_deserializer