Replace lua-rapidjson with lua-cjson

[mckaydavis]

I propose replacing the lua-rapidjson package dependency with lua-cjson.

Rationale:

• One less dependency when using wiola within OpenResty as lua-cjson is a default component in OpenResty -- while lua-rapidjson is not.
• Furthermore, installing lua-rapidjson via luarocks install rapidjson is further complicated as that requires a compiler and CMake >= 3.0.0 to be installed to build lua-rapidjson from source.
• Minor point, but the resulting cjson.so binary is 95% smaller than the rapidjson.so binary:

  $ du -sb cjson.so rapidjson.so
  132100  cjson.so
  2444310 rapidjson.so

• For the API that wiola uses (i.e. .encode and .decode) it is a straight drop-in replacement with minimal code change. My preliminary testing after query-replacing all require "rapidjson" -> require "cjson" in src/wiola/wiola.lua seems to work fine.

Thoughts?

[KSDaemon]

Hi @mckaydavis (again :) ) Well, in the begining i was using cjson, but than i had switched to rapidjson due to lack of support of empty arrays. In cjson you can globaly configure to encode lua empty tables as objects or as arrays. But wiola needs to encode different empty tables differently. I've got a look into open resty fork, and it has a empty_array and empty_array_mt things, which is good replacement for rapidjson setmetatable({}, { __jsontype = 'object' }) Will be great, if you can make a PR with that ;)

[KSDaemon]

And another one reason for using rapidjson — *cjson lacks support for big integers (> 32 bit)