Include RapidJSON headers only if BOND_SIMPLE_JSON_PROTOCOL defined

[oliora]

Correct me if I'm wrong but RapidJSON headers needed only for simple JSON protocol. But this headers are included in any case. This forces a user to has a dependency on RapidJSON even if he/she is not going to use JSON at all.
I propose to include simple_json_xxx headers (along with RapidJSON headers) only if BOND_SIMPLE_JSON_PROTOCOL is defined.

[chwarr]

Yes, this makes sense.

[sapek]

The boost::variant used in the implementation of bonded<T> always has all protocols, even disabled ones. The reason for this was to minimize potential for catastrophic, potentially exploitable and very hard to diagnose errors when ODR rule violation would lead to wrong layout of the internal variant data.

[oliora]

I've checked the code. The bonded type that minded as protocol agnostic depends on all protocols. That's really strange for me. How this approach will allow to add new protocols without breaking binary compatibility with old code?

[chadwalters]

Revisiting this old issue. Many of the ODR-related concerns and the issue raised above about bonded<> are now addressed as of #425. However, SimpleJSON still appears in the list of BuiltInProtocols. I think that this would be a fairly impactful breaking change for some of the heavier users of Bond internally.

[chwarr]

Agreed about the breakage. RapidJSON could be made opt-out pretty easily now, though.