Implemented support for recursive $ref

[milux]

This PR implements parsing and codegen for recursive $refs.
It requires pwall567/json-kotlin-schema#20 or another patched version of json-kotlin-schema that properly creates cyclic schema objects.

The cycle detection strategy is not very pretty, yet effective.
I tried to implement it as efficiently as possible. Each affected function call along the tracked call chains will have one more reference parameter. Apart from the check for the existence of an element in the call chain, which naturally is O(n) with n being the depth of the relevant call stack, the add/removeLast ops are all O(1).
Using HashSets/HashMaps may further improve performance for deep call stacks due to all ops being O(1) there, but due to higher costs of each operation I considered simple (Array)Lists the better option.

[milux]

PR is still relevant, but base changed to https://github.com/milux/json-kotlin-schema-codegen/tree/feature/recursive-refs
Edit: Thought that renaming the branch in my repo for clarity would also cause changing the base branch here. This obviously didn't work. If desired, I can open a new PR with the right pointers.

[milux]

Tracked in #46