OpenResty type annotation updates

[flrgh]

changelog

• Add cjson and cjson.safe
  • OpenResty ships with its own fork of cjson that includes some extra features
• Add string.buffer
  • this is actually a core luajit extension, but it's relatively new, so I don't know if it's included in other mainstream LuaJIT distros right now.
  • https://github.com/openresty/luajit2/blob/v2.1-agentzh/doc/ext_buffer.html
• Update resty.dns.resolver
  • https://github.com/openresty/lua-resty-dns

[sumneko]

Thank you!