Tracking Issue for core rework

[obmarg]

I'm currently refactoring the internals of cynic pretty heavily to move logic out of the use_schema output and into QueryFragment. This issue is to track the progress of that work:

• [x] Prototype new API
• [x] Refactor use_schema
• [x] Refactor derive(QueryFragment)
  • [x] Basic query functionality
  • [x] Enums & Custom Scalars
  • [x] Spreads
  • [x] Variables
  • [x] Interfaces & Union Types
  • [x] Flatten attr
  • [x] Recursion
  • [x] aliases
• [x] InlineFragments derive
• [x] Enum derive
• [x] Scalar derive
• [x] Argument handling
• [x] FragmentArguments derive
• [x] InputObject derive
• [x] impl_scalar!
• [x] Query building
  • [x] Define API
  • [x] Implement Query Builder
  • [x] Arguments
  • [x] Aliases
  • [x] Inline fragments
  • ~Named fragments (a nice to have really)~
• [x] Deal with any todo!s
• [x] Update generator (Need to figure out how to keep it backwards compatible...)
• [x] Get all tests passing
• [x] Bump the version number
• [x] Merge #391

Can merge at this point, and then future work:

• [x] Benchmarks
  • [ ] ~Building Queries~
  • [x] Decoding Responses
  • [x] The compile of dependant projects or the runtime of the macros themselves
• [x] #396 (doing this one later)
• [x] #417
• [x] ~Figure out whether we can do borrowed arguments.~ Don't think so
• [x] #419
• [x] #418
• [x] Tidy up mess
• [x] Optimised inline fragments (WIP here)
• [x] Re-instate subscriptions & streaming operations
• [x] Fix up InlineFragments typesafety, enforce fallbacks etc. (#442)
• [x] A test of InlineFragments fallback decoding (on both interfaces & unions, with unit & newtype variants) (#443)
• [ ] More thorough UI tests (probably just make an issue for this
• [ ] Test with downstream projects
• [ ] Document upgrade process
• [ ] Update book
  • [x] Update the section on the derives (https://github.com/obmarg/cynic/pull/432)
  • [ ] Update the bit on manually writing queries.
  • [ ] Update the reference docs if they need it?