[Merged by Bors] - Refactor much of the core of cynic

[obmarg]

Why are we making this change?

There are a number of enhancement issues in the tracker that would be difficult
to do with the current core of cynic:

• Stop relying on serde_json for decoding #249 is difficult because json_decode is very tied to serde_json. The
  runtime decoding setup in cynic makes it very hard to integrate with serde
  directly, which would be the easiest way to acheive this.
• Optimise for large schemas #150 wants to optimise cynic for larger schemas. The use_schema macro
  seems to be the main cause of the slow down, mostly as a result of the
  quantity of code it outputs. With the current cynic setup it's difficult to
  fix this.
• Better decoding errors #186 wants to improve the decoding errors. There are ways to do this with
  serde directly, but with json_decode it's hard.

What effects does this change have?

Far too many changes for a single PR:

1. Removes the dependency on json_decode:
   • All output types now need to implement serde::Serialize
   • All input types now need to implement serde::Deserialize
   • All the cynic derives should derive correct versions of these traits.
2. The use_schema output has been completely reworked to work with a new
   typesafe query builing API. This should cut down drastically on the output
   for large schemas (not sure it'll help enough, but it's a start).
3. Added a new format for argument literals - they now accept a format very
   similar to GraphQL argument syntax and this will be converted into literal
   GraphQL in the query string. Previously any arguments would have been
   serialized as variables in the eventual query.
4. Renamed FragmentArguments to QueryVariables

Fixes #249
Fixes #250
Closes #32 (no longer needed)

Still To Do

• Update CHANGELOG

[bors]

Pull request successfully merged into master.

Build succeeded:

• build-rust
• build-ts