refactor fatcat API server (fatcatd, Rust) to use OpenAPI Generator 5.x and hyper 0.13+

[bnewbold]

This will likely involve switching the API schema definition itself from "swagger" (openapi 2) to openapi 3.

fatcatd uses a generated API interface library created using an old and deprecated version of the swagger code generator. Some of the problems with this situation:

• generator tool is old enough that hard for others to run (eg, docker image no longer online?)
• lots of lint warnings and other problems in generated code that we work around with automated search/replace
• creates tokio/hyper dependency conflicts

OpenAPI Generator 5.0 has been released. The new output server schema/structure uses hyper 0.13 directly, instead of via the iron framework. We should refactor to use this generator, and the hyper server/service API, without iron. This API is async; we can either use threading (as we do currently), or have an async structure with thread pool workers wrapping blocking calls (like diesel database queries).

[bnewbold]

Partial progress on this (eg, openapi3 yaml and codegen stuff) in: https://gitlab.com/bnewbold/fatcat-cli

To be clear, intend to maintain fatcat-openapi crate in this repository (fatcat). fatcat-cli will be able to just depend on a published crate once we have a 5.0 version for use with fatcat API server, and we can rip the codegen stuff out of the fatcat-cli repo.